As others have said - nope, absolutely not, never heard anyone do that before and most certainly not AFTER having sent out invites.. Slightly different story if they sent out the invite with a:
"As close friends you'll all know and understand things are a little tight for us at the moment, having said that we really want to get all our friends and family together so hope you won't mind if we ask if everyone would mind making a contribution towards the meal rather than buy presents, something in the region of £20 per person would be really helpful. If you would rather not join us for a meal then we'd love to see you in the dog and duck for some drinks and a dance later in the evening."
"As close friends you'll all know and understand things are a little tight for us at the moment, having said that we really want to get all our friends and family together so hope you won't mind if we ask if everyone would mind making a contribution towards the meal rather than buy presents, something in the region of £20 per person would be really helpful. If you would rather not join us for a meal then we'd love to see you in the dog and duck for some drinks and a dance later in the evening."