"It is not expensive!"
Translation:Níl sé costasach!
daor is another word that you can use - broadly speaking, you can equate daor to "dear" and costasach to "costly", and you can usually use either of them for "expensive", but there are certain contexts where one or the other might be more appropriate.
(Daor also means "un-free" in the sense of freedom or liberty, it can be used for "serf" or "slave". It would be inappropriate to use it if you were talking about "expensive craftsmanship", for example).
Costasach is just an adjective. It is describing the subject of the sentence ("it" or sé). Adjectives typically come after the noun/pronouns that they describe.
Níl sé costosach.
If the sentence was "it's not a box", using a noun (box) rather than an adjective (expensive) you would be using a copula rather than the verb bí, and the negative form would be ní rather than níl, and you could say ní bosca é, but that's a different grammatical structure.
All present tense verbs are negated by putting ní in front of them:
ní chreidim thú, ní thuigeann sé, ní ólaimid caife, ní itheann sí mórán.
In the case of the verb tá, a special form fuil is used after verbal particles like an and ní. The question form is an bhfuil? (an eclipses), and the negative form would be ní fhuil (ní lenites), but because fh is silent, ní fhuil is pronounced níl, and that is how it is now written.
So you alays use ní, except where the positive form of the sentence would use tá. If the positive version of the sentence would use tá, then the negative is níl.
The positive version of this sentence would be tá sé costasach so the negative form is níl sé costasach.