It sounds to me like you're being pretty clear that the money is not going towards sexual favours, but rather to cover costs like beverages and clean-up.
It would be very difficult for anyone to argue that you're doing anything illegal here, especially since there are other club nights of this nature that operate and charge fairly high admission fees all over the country!
Just to be extra sure, you may want to retain any receipts of expenses incurred, such as the cost of the drinks.
You also want to be careful that you're not breaching another law: Selling alcohol without a license.
Another idea would be to clearly make sure that any request for money is clearly labelled as a donation and not obligatory. Obviously if you invite a ton of cheapskates then you may make a loss, but doing this should protect you from any problems.
