You should probably post this in the smoking section, instead of the growing section. As for cost, it really depends where you live.
Med marijuana shops charge up to $80 a gram for good bubble hash. As for the regular stuff I don't know.
you mean how much is hash to buy? or how much leaf is needed to sieve into hash after harvest? for about a gram of hash, id say you need (depending on concentration of thc crysals) about two ounces of leaf. obviously if you're sivving bud you will need a lot less than sivving fan leaves. the small leaves trimmed from the buds work best i think, im always reluctant to go sivving away sweet sweet bud...