Is your tip supposed to be your actual BILL or...?

Your server is working just as hard for you as they would be if you were paying full price. So you should tip them as though you are paying full price. Anytime I use a coupon or discount on my bill I tip the difference, and my bill is still cheaper because the discount is the tip. (I get a free appetizer for $8..tip at least $8...and I'm still paying less than I would be without the discount after tip.) But that's coming from a server who knows how much it sucks to get screwed over by coupons. :p