nummos would be the accusative, but in this case we need an ablative! I don't think the ablative has been covered in the lections yet, but we can't really ask "/what/ does the wine cost", instead it's "/how much/ does the wine cost". this is one but many uses of the ablative, but i don't remember which rules exactly apply to it, so i won't give you any. i'm sure the course will cover it!
consto is first conjugation (with the infinitive -are)
For the usage please see: II, B, 6. http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0059:entry=consto
Well, that's the problem with vocabulary that doesn't translate. Nummus is nummus, but that doesn't help new learners. We translate nummus and denarius and all other coin words as coin. We do accept nummuses and nummi.
EDIT: And no, they were not limited to the Byzantine Empire.