"The price of the kitten is thirty euros."
Translation:La prezo de la katido estas tridek eŭroj.
Are you sure, in the tips it says that the -n (in addition to being used for the accusative) is added to measures and amounts. I didn't really get how that works but I thought it might apply here.
If I understand correctly: -n ending is indeed used for measures, but not when the measure is the subject of the sentence or the description of the subject (in short, after esti).
So here, la prezo estas tridek eŭroj without -n ending, but with kosti, the sentence would become: la katido kostas tridek eŭrojn.
Edit: Duolingo is full of exceptions such as tiu fadeno estas unu metron longa. So, I really am not sure of what I just wrote!...
You'll note that your "exception" contains an adjective that is being quantified; if someone had said, La longo de tiu fadeno estas unu metro, there would have been no accusative.
"one metre long", "three kilogrammes heavy", "two inches wide" etc. would use accusative in the quantifier.
I've never heard anyone say "This book was ten euros expensive", but if one did say it, I'd expect the accusative in the measure word there as well (dek eŭrojn kosta or something like that perhaps).
Ĉu ne eblas "kosto" anstataŭ "prezo"? Ĝi havas sencon, ĉar "kosti" estas la verbo por esprimi kostojn. Ĉu ("prezo" anstataŭ "kosto") ne estas "anglismo"? Saluton
First time I did this one, I wrote "tricent" instead of "tridek".
"This kitten better be worth it..."
Shouldn't this be eŭrojn? Since we are speaking of cost here there should be the added -n ending. As explained in the Tips & Notes.