"It is not expensive!"
Translation:Níl sé costasach!
Assuming you were already discussing a feminine object, why would Níl sí costasach be wrong?
Generally you just use "sé" for it, unless you are specifically talking about a feminine object.
Please remind me what "ea" was for, thought that it was like it? while sí or sé were with a gender...
I thought "daor" existed as a word for expensive. Is this word taboo? Is the De Bhaldraithe English-Irish dictionary wrong?
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.