El precio fijo no cambia; en cambio, el precio indexado sí.

Here sí is being used to stand in for the omitted verb phrase, a lot like English does in the indexed price does.

So:

• El precio fijo no cambia = the fixed price does not change
• el precio indexado sí = the indexed price does

It avoids repeating cambia. A fuller version would be:

• El precio fijo no cambia; en cambio, el precio indexado sí cambia.

Both are correct, but the shorter version sounds more natural because Spanish often leaves out repeated material when it is obvious.

Because sí with an accent is the affirmative word meaning yes or indeed / does in this kind of contrast.

Without the accent, si usually means if.

So these are different words:

• sí = yes / indeed / does
• si = if

In your sentence, it must be sí because it is affirming the contrast: the fixed price does not change, but the indexed price does.

En cambio means something like by contrast, on the other hand, or whereas. It is especially useful when comparing two things in a clear, balanced way.

So:

• pero = but
• en cambio = by contrast / on the other hand

In this sentence, en cambio works well because the two prices are being directly contrasted:

• fixed price: no change
• indexed price: change

You could say pero here, but en cambio sounds a bit more explicit and structured.

Spanish often repeats the noun for clarity and balance, especially in comparisons.

So this full version:

• El precio fijo no cambia; en cambio, el precio indexado sí.

is very natural.

You could also shorten it to:

• El precio fijo no cambia; en cambio, el indexado sí.

That is also possible, because precio is understood. But repeating el precio sounds clearer and more formal, especially in writing.

Because the subject is singular:

• el precio fijo = the fixed price

Since precio is singular, the verb must also be singular:

• el precio ... cambia

If the subject were plural, you would use cambian:

• Los precios fijos no cambian.

The present tense here expresses a general fact or typical behavior.

• no cambia = does not change
• sí = does / does change

Spanish often uses the simple present for statements that are generally true, habitual, or valid now. English does the same in this kind of sentence.

Indexado is an adjective meaning indexed or linked to an index. In economic or financial contexts, it usually means that the price changes according to some reference index, such as inflation.

So:

• precio fijo = fixed price
• precio indexado = indexed price

It is a more technical word than everyday vocabulary, but it is common in business, finance, contracts, and energy pricing contexts.

Yes. That is a fully correct version.

Compare:

• El precio indexado sí.
• El precio indexado sí cambia.

The first one omits cambia because it is already understood. The second one keeps the verb for extra clarity or emphasis.

Also note the position of sí:

• El precio indexado sí cambia = the indexed price does change

Here sí is placed before the verb to emphasize the contrast.

Because cambiar can work as an intransitive verb meaning to change.

Here:

• El precio fijo no cambia = the fixed price does not change

This means the price itself does not vary.

If you said se cambia, that would usually suggest a different idea, such as something being changed or replaced, depending on context. That is not the meaning here.

So cambia is the natural choice for changes in the sense of varies.

The semicolon links two closely related clauses while marking a stronger pause than a comma.

Here it works well because the sentence has two balanced parts:

• El precio fijo no cambia
• en cambio, el precio indexado sí

The semicolon highlights the contrast neatly. A full stop would also be possible, and sometimes a comma is used, but the semicolon is a very good choice in careful written Spanish.

Yes, but the original order is very natural and clear.

For example, you could also say:

• El precio fijo no cambia; el precio indexado, en cambio, sí.

This places en cambio inside the second clause instead of before it.

You could even say:

• En cambio, el precio indexado sí cambia.

if the first idea had already been mentioned before.

So the exact position of en cambio can vary, but the original sentence is a standard, elegant way to express the contrast.