In this day and age where wedding costs are going through the roof and most couples already have everything they need, it's not out of line to ask the guests to contribute to their meals instead of bringing a gift.
I have created a few invitations in the past where the couple has made this request.
I recently had a bride who did this for her wedding. In her invitations, she included a menu where you could pre-order your dinner on the RSVP card. Her wording was as follows:
"As most people would normally have a wishing well or gift table, we are requesting something different!
Since we are combining two households into one, we have no need for anything.
Your gift to us is just your presence and for you to purchase your own meal.
We have included a menu for the night and would appreciate if you could please pre-order when you RSVP.
This will make the night run smoothly. If any issues arise regarding the meals or accommodation, please let us know and we will help in any way possible."
If it's just a standard cost per head, & guests can't choose what they want ahead of time, you can amend it to something along the lines of "we respectfully ask for a contribution of $x per head for your meal" & either provide your bank details or ask for cash on the night.
Good luck and let me know what you end up writing :)