The price that you charge for the weekend has got to cover accommodation for the CU members, transport costs and food. It has also got to include the accommodation and travel costs of speaker(s) and an appropriate gift for the time they have spent in preparation. The centre may not have included VAT in the price they quoted you (check this) and there are also costs for the programmes and publicity, stationery and stamps and possibly for any phone calls made by the houseparty organiser. It is therefore vital that you work through your budget carefully and crosscheck it with somebody else before going public with a figure that simply will not cover your outlay. You might find it helpful to add an extra 10% to your finally calculated figure as a cushion against unforeseen circumstances. This may also allow you to subsidise those who would like to come on the weekend but have genuine financial difficulties.