Have you tried the Squash club on Manse Lane? My friend had her 30th there, don't know how much it was but don't think it was too pricey. There's a bar so don't have to worry about supplying drinks or telling people to bring their own and you can cater yourself so can cut costs there, my friend just cooked loads of sausage rolls, cheese and pineapple sticks etc..
Mind's drawn a blank at the mo, will get back to you if I remember anymore :o)