Какое бы платье ни выбрала моя подруга, оно всё равно будет стоить дорого.

This is a very common Russian pattern meaning no matter which..., whichever..., or whatever... depending on the noun.

So:

Какое бы платье ни выбрала моя подруга...
= Whichever dress my friend chooses...
= No matter which dress my friend chooses...

The full sentence means something like:

No matter which dress my friend chooses, it will still be expensive.

This pattern is often built like this:

• какой / кто / где / когда / сколько etc.
• • бы
• • the rest of the clause
• • ни
• • verb

Examples:

• Куда бы мы ни поехали, везде жарко.
  = Wherever we go, it’s hot everywhere.

• Что бы он ни сказал, я не поверю.
  = Whatever he says, I won’t believe him.

Because together they create this special concessive / free-choice meaning: no matter which, whoever, wherever, etc.

In this sentence:

• какое бы helps introduce the idea of an unspecified choice
• ни выбрала completes the pattern

So какое бы ... ни выбрала does not mean a normal negative sentence. It means:

regardless of which one she chooses

This is a fixed and very common structure in Russian. You should learn it as a whole pattern:

• кто бы ни... = whoever
• что бы ни... = whatever
• где бы ни... = wherever
• как бы ни... = however / no matter how
• какой бы ... ни... = whichever / no matter which

No. That is a very common confusion.

Here, ни is not making the verb negative by itself. The sentence is not saying she didn’t choose.

Instead, ни is part of the whole construction:

какое бы ... ни выбрала

So the meaning is concessive, not negative:

• Какое бы платье ни выбрала... = Whatever dress she chooses...
• not Whatever dress she doesn’t choose...

If you wanted a real negative with the verb, you would normally use не, not ни:

• Она не выбрала платье.
  = She didn’t choose a dress.

Because they agree with different words.

• какое agrees with платье
• выбрала agrees with моя подруга

1. какое

Платье is:

• neuter
• singular

So the word referring to it must also be neuter singular:

• какое платье

2. выбрала

The verb in the past tense agrees with the subject, which is моя подруга.

Моя подруга is feminine singular, so:

• она выбрала
• therefore моя подруга выбрала

That is why the sentence has both:

• какое = neuter, matching платье
• выбрала = feminine, matching подруга

Because платье is a neuter singular noun, and here it is the direct object of выбрать.

For an inanimate neuter singular noun, the accusative form is the same as the nominative form.

So:

• nominative: какое платье
• accusative: какое платье

That is why you see какое, not a different-looking accusative form.

Compare:

• какой стол → какой стол
  masculine inanimate, accusative = nominative

• какую книгу
  feminine inanimate, accusative is different

So if the noun were feminine, the sentence would look different:

• Какую книгу ни выбрала моя подруга...

Because моя подруга is the subject of выбрала.

The subject of a Russian finite verb is normally in the nominative:

• моя подруга выбрала

So:

• моя = nominative feminine singular
• подруга = nominative feminine singular

If you change the subject, the verb changes too:

• мой друг выбрал
• мои друзья выбрали

Всё равно means all the same, anyway, still, or regardless.

In this sentence it emphasizes that the result does not change:

оно всё равно будет стоить дорого
= it will still be expensive
= it will cost a lot anyway

So the idea is:

No matter which dress she chooses, the outcome is the same: it will be expensive.

You will often hear всё равно in everyday Russian:

• Я всё равно пойду.
  = I’ll go anyway.

• Это всё равно ничего не меняет.
  = That still doesn’t change anything.

Because the first part is a subordinate clause and the second part is the main clause.

Structure:

• Какое бы платье ни выбрала моя подруга,
  subordinate clause

• оно всё равно будет стоить дорого.
  main clause

Russian uses a comma here just as English often does with:

Whichever dress my friend chooses, it will still be expensive.

So the comma marks the boundary between:

1. the no matter which... clause
2. the main statement

Оно refers back to платье.

Since платье is neuter singular, the pronoun must also be neuter singular:

• платье → оно

So:

Какое бы платье ни выбрала моя подруга, оно...
= Whichever dress my friend chooses, it...

Russian could sometimes omit the pronoun in some contexts, but here keeping оно sounds natural and clear.

Because the sentence is talking about a future result: after she makes her choice, the dress will cost a lot.

Стоить is an imperfective verb, so its future is formed with быть:

• стоить = to cost
• будет стоить = will cost

So:

• оно будет стоить дорого = it will cost a lot / it will be expensive

If you said стоит, that would usually describe the present:

• Это платье стоит дорого.
  = This dress is expensive / costs a lot.

Because дорого here is an adverb used with the verb стоить.

• дорогой / дорогое / дорогая = adjective: expensive
• дорого = adverb: expensively / at a high price

Russian often expresses price with the verb стоить plus дорого / дёшево:

• Это стоит дорого.
  = This costs a lot / is expensive.

You could also say:

• Это дорогое платье.
  = This is an expensive dress.

But that is a different structure:

• стоить дорого = to cost a lot
• дорогое платье = an expensive dress

Yes, that variant is also possible:

• Какое бы платье ни выбрала моя подруга...
• Какое бы платье моя подруга ни выбрала...

Both are grammatical.

The version with ни выбрала моя подруга places the verb earlier.
The version with моя подруга ни выбрала keeps the subject closer to the verb in a way that may feel more straightforward to learners.

Russian word order is flexible, but not random. Speakers move words around for rhythm, emphasis, and style.

In this sentence, both orders mean the same thing:

Whichever dress my friend chooses...

Yes, but it would not be exactly the same structure.

For example:

• Любое платье, которое выберет моя подруга, будет стоить дорого.

That also means something like:

Any dress that my friend chooses will be expensive.

But the original sentence with какое бы ... ни... sounds more like:

no matter which dress she chooses

So the original has a stronger sense of the choice does not matter.

Compare:

• Любое платье... = any dress...
• Какое бы платье ни... = whichever dress / no matter which dress...

This is another very common learner question.

In Russian, the form выбрала is grammatically past tense, but in this type of construction it often works in a way that English translates with a present meaning:

• Какое бы платье ни выбрала моя подруга... = Whichever dress my friend chooses...

Russian frequently uses past-tense forms in these бы ... ни constructions where English uses present tense.

So it is best not to translate word-for-word here. Instead, treat the whole clause as a pattern with the meaning:

whatever / whichever she may choose

The dictionary form is выбрать = to choose, to select.

Выбрала is:

• past tense
• feminine singular
• perfective

Why perfective? Because choosing a dress is viewed as a completed selection: she picks one dress.

Compare:

• выбирать = to be choosing / to choose repeatedly / to choose in process
• выбрать = to choose, to make the choice

In this sentence, perfective sounds natural because the idea is:

once she ends up choosing one, that dress will still be expensive

It is mostly neutral standard Russian. The pattern какое бы ... ни... is common and educated, but not unusually formal.

You can see it in:

• everyday speech
• writing
• journalism
• literature

So it is a very useful structure to learn. It may sound a little more polished than the simplest conversational alternatives, but it is absolutely normal Russian.