I side with the poster who suggested 50p.
It's unspeakably rude to ask for cash, there is no polite way of doing it.
And guests should not be asked or expected to cover all or part of the cost of a wedding.
I may desire a Lexus but if all I can afford to have is a Mondeo then that will just have to do.
If they want a wedding and are so financially strapped they should accept that they won't have the wedding of their dreams, and have a smaller guest list, cheaper meals, or a different venue.
I'm not convinced that being invited to a wedding is such a special priviledge as many bride and groomzillas seem to think, sure you may have a good night and have fun but there are many other types of parties one could attend.
You get invited to a wedding, you have to pay for new clothes, gifts, transport,parking, possibly accomodation and a babysitter.
Not such a good deal for the guests after all.