All the dictionaries and grammars I have consulted only talk about how to say that something is "more [adjective]." Would it be unnatural in Irish to say that something is "less [adjective]," for example, "níos lú costasaí" (less expensive)? Grammatically, is it correct to simply insert "níos lú" between the noun and the adjective in the comparative form? Conversationally, I could just paraphrase and say that something is cheaper, but it adds complications when writing poetry.

Mar a scríobh an t-athair Peadar: “Na daoine is lúgha ciall i n-Éirinn: Daoine gan Bhéarla gan Ghaeluinn!”
Na daoine is lú ciall: people that (their) sense is smallest, i.e. the least reasonable people.
O'Nolan, Gerald (1934), New Era Grammar of Modern Irish, p. 117 (https://archive.org/details/the-new-era ... canned%29/):
Ní fheaca riamh aon bhean ba lúgha tuisgint ná í — I never saw a less intelligent woman [than her].
Father O'Nolan calls it the accusative of specification, and writes more about it in his grammar on pp. 106 ff.

So one way to phrase your sentence would be, maybe: níos lú costas.
Similarly, I think: an rud a chosnaíonn níos lú ná an t-anam: the thing that costs less than life.
The problem is that I've just come up with the above and I don't know how "natural" or "good" those are, even if they are, perhaps, "technically correct".

You can’t use ‘níos lú/is lú’ with adjectives - only with a noun or a verb, so you have to re-phrase it in Irish. Have a look at https://focloir.ie/ga. Enter ‘less’ in the search box.

is lú + noun
bean is lú áille (less beautiful woman)
is lú solas (less bright)
is lú costas (less expansive)

is lú is + adjective
Somewhere this was promoted for less + adjective
But it seems rather confined with only a few words: gann, féidir, gá in modal constructions
e.g. is lú is gann dom é sin a dhéanamh (gann, féidir gá aren't really adjectives but nouns)
Rarely with other adjectives

níos lú + adjective
I doubt this is correct, "níos lú gar" the only example I found.