Learn Hardcore Turkish: Fiyat sabit.

In Turkish, in the simple present tense, the copula (“is/am/are”) is omitted when you have a noun or adjective predicate. So instead of saying Fiyat sabit +"is," you just say Fiyat sabit. It literally reads “Price fixed.”

Why don’t we see “the” before fiyat? Shouldn’t it be “The price is fixed”?

Turkish has no articles (“a,” “an,” “the”). Definiteness and indefiniteness are determined by context or by adding suffixes. Here, it’s clear we’re talking about a specific price, so no article is needed.

Why is the word order Fiyat sabit (noun + adjective)? I thought adjectives come before nouns.

That rule applies to attributive adjectives (when they directly modify a noun), e.g. sabit fiyat (“fixed price”). In a predicative sentence (with “is”), the adjective follows the noun: Fiyat sabit (“the price is fixed”).

How would I make this sentence negative?

Add değil after the adjective:
Fiyat sabit değil.
This means “The price is not fixed.”

How do I turn it into a question (“Is the price fixed?”)?

Simply add the question particle mi (which agrees in vowel harmony and spacing) at the end:
Fiyat sabit mi?

Can sabit change for number or gender, like “fixed prices”?

No. Turkish adjectives are invariable. To say “fixed prices,” you pluralize fiyat:
Fiyatlar sabit.
The adjective sabit stays the same.

How do I use this adjective attributively, as in “There is a fixed price”?

Place sabit before fiyat:
Sabit bir fiyat var.
Here bir (“a”) makes it indefinite: “There is a fixed price.”

Are there other words similar to sabit I could use?

Yes, depending on nuance:

değişmez (“unchangeable”)
kalıcı (“permanent”)
So you might hear Fiyat değişmez or Fiyat kalıcı, though sabit is the most common.