"How much is this?"
We don't know what the "this" referred to in the sentence is - when gender is unknown, you default to the masculine.
Should "Quant'è?" be accepted? I learned this in a phrase book and believed it to mean the same thing
The first time i see a question I try to answer in the best Italian I can muster. If I'm completely stumped I practice my Japanese. "Ikura desu ka?" If I'm going to be wrong, I'll get some good out of it anyway.
should "Quanta è questa" also be accepted? I figured it would be okay as long as the endings matched... apparently not?
you have to understand the root of it. "Quanto" refers to "the price of the thing" and it's masculine: "Quanto è?". There's no way around it. Now, as for the second half, you are going to ask "this", but in english it has no gender. In Italian, Spanish or Portuguese it has, because you are asking the price of a definite object, of which you know the name of and its gender so, you will have a word (feminine or masculine) for stating the particular object out: questO (this masculine object) or questA (this feminine object). Quanto è questo? Quanto è questa? "Quanto è" will always be masculine because it refers to the price and not to the object itself ;)
I hope I could be of use