I don't see anything wrong with them asking for $, but it's also fine for anyone to decline the invite because of that. However, my closeness to the person hosting or asking, would be the determinate as to whether or not I'd go, not the asking for $
I think it is inapprorpriate to ask them for money or any gift.If you get one, that's great. If not, it is still great because you want to share a special event wth your friends and family members. Or you could say no gift or instead of a gift, make a donation to a worthy cause.