The problem with making the prizes too good is that a big cash prize can attract a “professional” team. And if a team of big hitters keep collecting the spoils the rest of the teams will lose interest, and probably decide not to come again.
A good scheme is to give the winners bar tokens, perhaps redeemable on your next quiz night. In that way they’ll be back next time.
If you limit the team size to six (and see another blog on team sizes) then six drinks will give a full team one each. And if the team had fewer players, they’ll get more drinks each.