La botella pequeña cuesta cincuenta céntimos menos que la grande.

In Spanish, most descriptive adjectives normally come after the noun.

So:

• la botella pequeña = the small bottle

This is the most neutral, standard order. You can sometimes put an adjective before the noun, but that often adds emphasis, emotion, or a more literary tone. For a basic physical description like this, botella pequeña is the natural choice.

Cuesta is the third-person singular form of the verb costar (to cost).

The subject is la botella pequeña, which is singular, so the verb must also be singular:

• la botella pequeña cuesta
• the small bottle costs

Other forms would be:

• cuesto = I cost
• cuestas = you cost
• cuesta = he/she/it costs
• cuestan = they cost

Because it agrees with botella, which is a feminine singular noun.

In Spanish, adjectives usually agree with the noun in gender and number:

• botella → feminine singular
• pequeña → feminine singular

Compare:

• el vaso pequeño = the small glass
• la botella pequeña = the small bottle
• las botellas pequeñas = the small bottles

In Spain, céntimo(s) refers to cents, especially euro cents.

So:

• cincuenta céntimos = fifty cents
• more specifically, 50 euro cents

In everyday Spanish in Spain, this is completely natural. You may also hear amounts expressed as:

• cincuenta céntimos
• cero con cincuenta euros
• medio euro in some contexts, though that is less direct here

The accent mark shows the correct stress: CÉN-ti-mos.

Without the accent, the word would not follow the expected pronunciation pattern. Written accents in Spanish are very important because they tell you where the stress falls and sometimes distinguish between words.

So:

• céntimos → stress on the first syllable

Because this is a comparison between two things:

• the small bottle
• the big one

In Spanish, when comparing, you normally use:

• más que = more than
• menos que = less than

So:

• cuesta cincuenta céntimos menos que la grande

By contrast, menos de is usually used with numbers, quantities, or limits:

• menos de diez euros = less than ten euros
• menos de una hora = less than an hour

Because Spanish often leaves out a noun when it is already understood from the context.

So:

• la botella pequeña ... la grande

really means:

• la botella pequeña ... la botella grande

But repeating botella would sound unnecessary. La grande means the big one here.

This is very common in Spanish:

• el rojo = the red one
• la barata = the cheap one
• los pequeños = the small ones

The article la is needed because grande is standing in for a noun.

So la grande does not just mean big. It means:

• the big one
• more literally, the big [bottle]

This structure is very common:

• el pequeño = the small one
• la azul = the blue one
• los nuevos = the new ones

Yes, that is also correct and natural.

There are two common ways to express this idea:

• La botella pequeña cuesta cincuenta céntimos menos que la grande.
• La botella pequeña es cincuenta céntimos más barata que la grande.

The meaning is basically the same. The first focuses on cost:

• costs fifty cents less

The second focuses on being cheaper:

• is fifty cents cheaper

Both are good Spanish.

Grande becomes gran only when it comes before a singular noun.

So:

• una gran botella = a great/big bottle
• la botella grande = the big bottle

In your sentence, grande comes after the noun it describes, so it stays grande.

Also, gran often suggests great in the sense of importance or impressiveness, not just physical size, depending on context.

Not entirely, but it fits Spain very naturally.

In Spain, since the currency is the euro, céntimos normally means euro cents. In other Spanish-speaking countries, people may also understand céntimos, but actual usage depends on the local currency and local habits.

So for Spanish from Spain, this sentence sounds completely normal and idiomatic.