Les fraises du marché sont chères, mais leur goût est magnifique.

Du is a contraction of de + le (of/from + the, masculine singular).

• de + le marché → du marché
• It literally means of the market / from the market.

In French, you cannot say de le marché; the language always contracts de le to du. So les fraises du marché is the strawberries from the market.

French usually needs an article in front of a noun, even when English does not.

• les fraises = the strawberries
• Dropping the article (∅ fraises du marché) is normally wrong in standard French (outside of some titles, lists, or telegram style).

So to talk about these strawberries in a normal sentence, you say les fraises du marché.

Adjectives in French agree in gender and number with the noun.

• une fraise is feminine singular
• les fraises is feminine plural

The base adjective is cher (expensive).
Its forms are:

• masculine singular: cher
• feminine singular: chère
• masculine plural: chers
• feminine plural: chères

Since les fraises is feminine plural, you must use chères.

The accent changes both spelling and pronunciation:

• cher: e is pronounced like é (closed sound, similar to ay in day but shorter)
• chère / chères: è is an open e sound (more like e in bed).

Feminine forms are written chère / chères with è, not chere / cheres.
So chères is the correct feminine plural spelling.

In French, être + adjective is enough:

• Les fraises sont chères. = The strawberries are expensive.

You do not add an extra word like are expensive ones.
The structure subject + être + adjective already makes a complete predicate.

Leur here is a possessive determiner meaning their.

• leur is used when the possessed thing is singular, even if the owners are plural
• leurs is used when the possessed things are plural

Here, the strawberries (plural owners) have one overall flavor:

• owners plural, thing singular → leur goût = their taste / flavor

You could say leurs goûts if you really meant several different tastes, but normally for food you talk about its overall flavor, so you keep goût singular and use leur.

Son goût would mean its taste / his taste / her taste, referring to a single owner, not to the strawberries as a group.

French often uses a singular noun when talking about a general quality shared by a plural subject.

The idea is: The strawberries’ flavor (as a whole) is magnificent, not each individual strawberry has its own separate taste.

So:

• Les fraises sont bonnes. → plural adjective for the berries themselves
• Leur goût est magnifique. → singular goût for the overall flavor they have.

Magnifique is an adjective that agrees with the noun it describes.
Here, it describes goût, which is:

• le goût → masculine singular

So the adjective must be masculine singular too:

• un goût magnifique
• leur goût est magnifique

If the noun were plural (for example leurs goûts), you would write magnifiques.

Yes, you can; the meaning shifts slightly but the grammar follows the same rules of agreement.

• délicieux (delicious) is masculine singular and masculine plural
• délicieuse / délicieuses are the feminine forms

Since goût is masculine singular, you would say:

• Leur goût est délicieux.

Only the adjective changes; the structure of the sentence stays the same.

Approximate pronunciation (French-style IPA, then a rough English guide):

• Les fraises du marché → /le fʁɛz dy maʁʃe/

  • les → lay
  • fraises → frehz (final s = z sound)
  • du → dy (like dü)
  • marché → mar-shay

• sont chères → /sɔ̃ ʃɛʁ/

  • sont → nasal sohn (the t is silent here)
  • chères → shair (final s silent)

• mais → /mɛ/ → meh

• leur goût est magnifique → /lœʁ ɡu ɛ maɲifik/

  • leur → between lur and lœr
  • goût → goo (final t silent, the û lengthens the sound a bit)
  • est → eh (here the t is silent)
  • magnifique → ma-nyi-feek

Spoken smoothly, you get something like:
Leh frehz dü mar-shay sohn shair, meh lur goo eh ma-nyi-feek.

You can say leur magnifique goût or leur goût magnifique, but the meaning changes slightly:

• leur goût est magnifique: you are making a statement about their taste (their taste is magnificent).
• leur magnifique goût: you are describing the noun more compactly (their magnificent taste), usually as part of a larger sentence.

For example:

• Les fraises du marché sont chères, mais leur magnifique goût me plaît beaucoup.
  → …but their magnificent taste pleases me a lot.

In the original sentence, leur goût est magnifique is the most natural way to make a clear, direct comment about the taste.