and who is going to write your name on their's? if no one, then you paid $20 extra for your friend [and your firend (the bride/bridegroom) will appreciate your friend]; and they would think that you just ate a "free" meal! in any way, you got a free meal, but a bad reputation. and if you plan to get a bad reputation, might as well don't give and have a free meal!
some people give just enough to cover costs (can range from $80-120 per pax now given the price inflation of banquet! hotels been rising the prices). others "return" the amount that was given for their own.