Problem with asking for money is as some people have said they can't afford much. However that would not have stopped them from buying a very thoughtful present that may not have cost much money, but may have cost them time or effort.
So I think asking for cash might make some of your guests feel a little awkward if they could not afford to give much.
footflaps, you would be surprised, people in the majority will 'want/feel they have to' give you stuff anyway. it may as well be stuff you actually want
Zokes:
"Unless you happen to be asking for it for any number of reasons, as detailed by many above.
Reading FAIL"
No, it's still weak. I outlined the only exception. I read the reasons and they were guff.
And using the term 'FAIL'? 2004 called. They want their meme back.