Ove farmerke su lepe, ali nisu jeftine.

Yes. Farmerke is normally used as a plural noun, just like jeans in English. Even when you mean one item of clothing, Serbian usually still says farmerke in the plural.

So:

• ove farmerke = these jeans
• te farmerke = those jeans

Because the noun is plural, the verb and adjectives must also be plural.

Because ove has to agree with farmerke in gender, number, and case.

Here, farmerke is:

• feminine
• plural
• nominative case

So the correct form of this/these is ove.

A useful comparison:

• ovaj kaput = this coat (masculine singular)
• ova haljina = this dress (feminine singular)
• ovo dete = this child (neuter singular)
• ove farmerke = these jeans (feminine plural)

Because adjectives in Serbian must agree with the noun they describe.

Since farmerke is feminine plural nominative, the adjectives must also be feminine plural nominative:

• lepe = beautiful / nice / pretty
• jeftine = cheap

So:

• lepa haljina = a beautiful dress
• lepe haljine = beautiful dresses
• lepe farmerke = beautiful jeans

The ending changes because of agreement, not because the meaning changes.

Su means are. It is the 3rd person plural present tense form of the verb biti (to be).

A few present-tense forms of biti are:

• sam = I am
• si = you are
• je = he/she/it is
• smo = we are
• ste = you are
• su = they are

So:

• Farmerke su lepe. = The jeans are beautiful.

In Serbian, the negative present forms of biti are special forms:

• nisam = I am not
• nisi = you are not
• nije = he/she/it is not
• nismo = we are not
• niste = you are not
• nisu = they are not

So nisu is simply the correct form meaning they are not.

That is why:

• nisu jeftine = they are not cheap

Because Serbian often leaves out subject pronouns when they are not needed.

The verb form already tells you who the subject is:

• su = they are
• nisu = they are not

So Serbian does not need to say one (they) here. The noun farmerke already makes the subject clear.

A full version with the pronoun would usually sound unnecessary here.

Lepe can cover several English words depending on context:

• beautiful
• pretty
• nice
• sometimes even good-looking

For clothes, lepe often sounds very natural as nice or pretty.

So Ove farmerke su lepe could be translated as:

• These jeans are nice
• These jeans are pretty
• These jeans are beautiful

The best English choice depends on tone and context.

Ali means but.

It connects two contrasting ideas:

• Ove farmerke su lepe = These jeans are nice
• ali nisu jeftine = but they are not cheap

Its placement is very similar to English:

• statement 1 + ali
  • statement 2

So this sentence structure is quite straightforward for an English speaker.

Yes, it is very natural.

Serbian word order is more flexible than English, but this version is neutral and standard:

• Ove farmerke = topic
• su lepe = comment about them
• ali nisu jeftine = contrast

You may see other word orders in special contexts for emphasis, but this sentence is a very normal way to say it.

Lepe is the Ekavian Serbian form. Lijepe would be an Ijekavian form used in some other standards and regional varieties.

So:

• lepe = standard Serbian in Ekavian pronunciation
• lijepe = Ijekavian equivalent

Both correspond to the same basic meaning, but if you are learning standard Serbian as commonly taught from Serbia, lepe is exactly what you would expect.

Farmerke is the normal everyday Serbian word for jeans in general. It does not only mean classic blue denim jeans, though that may be the most typical image.

So ove farmerke simply means these jeans. The sentence itself does not specify color, style, or brand.