얼마나 될 거예요? is needlessly complicated.
We say 얼마예요?, which is like "How much is it?".
Of course the ending can be different, as in 얼마지요?, 얼마입니까?, etc. depending on the situation.
Yeah, I forgot that it was about big items like a hour or car. You are right you wouldn't say "How much is it?" for those :-)
To just get the general idea on the price, you could ask (가격이) 얼마나 할까요?/될까요?, (돈이) 얼마나 들까요?, (가격을) 어느 정도로 잡아야 될까요? The first two are like "How much would it amount to?", and the last one's similar to "What sort of price am I looking at?".
되다 and 하다 in these have the sense of "amount to" or "come to".
될 거예요? sound a little unnatural though. A similar but more natural way to say it is 얼마나 될 것 같아요? (What do you think the price will be?).