you are maying for the pipers "time" a lot of time at weddings you are there for several hours but out of those few hours you might actually be playing for 30 -40 minutes spread over the few hours.

but youve got factors to take into account

cost of fuel
cost of a set of pipes
cost of uniform
preparation
cost of the pipers time
experience

when you book a piper dont go for price, try and get a few referrals, the piper doesnt have to be a gold meddalist, just a solid steady blower, nice tone and a good range of tunes, also a nice person a lot of guests like to "meet and greet" the piper, have photos and stuff
the piper is there to entertain, I have a friend who pipes very well, however he will play his pipes , then drink the bar dry, hit on all the women and invite them to check his "underwear status" monopolise the dance floor and snarl at small kids that get in his way!