I think its ok to ask people to buy their own drinks - we did this too as we really couldnt afford to have that extra expense also. We donated $100 to the church, i made the cake myself which probably cost me $40 in ingredients and decorations (fed 60 people). $300 on food at the hotel function room, gown was my dh's. I bought myself a new dress $50.
So we spent around $500