I generally give cash as a wedding gift, and try to give enough to sort of "cover" my meal.
In my area, most banquet places charge at least $60-$75 a head for a meal, so I usually give at least that per person(if DH and I get invited, I go $150. If I am invited alone I go $75).
Of course, if it's someone I feel closer to, I may go more.
That's the geneal rule of thumb I use.