it all depends on your guests...
we had one for our engagement party - there were about 3 beers (pints/half pints of local boutique beer, not bottled beers), 2 whites, a red, sparkling and soft drink and coffee. the wines were about $25-30 a bottle. We had about 80 people for a cocktail function and planned on initially putting $1500 on the tab with the option of adding more. We didn't even get through the first $1500..i think our tab came to $1100!!
i was surprised - everyone seemed to be drinking something and quite a few of the boys were drinking quite a lot of beer!
we are also having one at our wedding - the tab will automatically be about $2500 as we have minimum spend to meet!