There's an idiom in German, "Was kostet der Spaß?", which translates directly to "quanto costa il divertimento?". However, it is not used to ask for the price of enjoying yourself in an almost philosophical manner as seems to be the case here. It is instead used to refer to the pricing of something, be it entertaining or not, with the "fun" often being sarcasm.
I know it is frustrating when a perfectly correct English translation is not accepted and you lose a heart, but to all those who complain about DL's rigidity I recommend trying studying Italian using Memrise, an excellent fun site but they accept only one "correct" answer. If the meaning of the Italian is ambiguous then complaints are in order but if you understand the meaning of the Italian, that's all that counts. Learn the answer DL expects and move on.
I would very much enjoy to see one of the DL creators to learn a new language starting from a foreign base language, not their native language... It would be so much fun to see them failing at the base language, even though they correcly understand the foreign language they currently learn. This would be priceless.
Yes that would be incorrect, I can't exactly tell you why but it sounds awful- something like 'What costs is the fun' instead of 'what is the cost of fun'. Quanto costa doesn't really translate well into English though. You could say- Quant'é il divertimento (How much is) instead :)