I dont think its appropriate to tell them. What i would suggest, if it is money you would like instead of gifts, let them know that. It is rather insulting to recieve an invitation and have the gist be, "oh and if you do come, you are costing me $50" Its not so insulting to ask them to give you money instead of gifts, although that may be insulting to some people.
We are meeting in a restaurant. I recall reading about letting each guest know in advance how about the stimated cost at the restaurant and on the top of it each guest pays for themself and tips somemore to cover for the bride...
