there's two questions you should ask yourself: how good of a trampoline do you want? and how much are you willing to spend?
if you want a complete competitive gymnastics setup, brand new, you're looking at about $6000 USD. that's frame, 7'x14' bed, springs, and mats. i recommend buying from reboundproducts.com - dave ross, canadian trampoline guru, makes the finest beds in the world. eurotramp setups run for about $5500, but ross beds are the way to go.
you'd be suprised at how often you can get good deals on competitive setups if you look hard too. i scored my nissen frame for free as a matter of fact. i went by a gymnastics club and was chatting with the director when she mentioned she had a old, kinda beat up frame, that she was willing to give away. i came by the next day to pick it up. it cost me $2000 CDN for a ross bed and ross springs, but it was well worth it.
my bro in kelowna looked in the buy and sell and scored a 5x10 (bed) tramp for $200 CDN. it's kinda haggard, but it's one of the nice foldup frames and is decent. he had to move and gave it to me instead. i now have two trampolines in my backyard and its a dream come true (i move into a new house with a pool soon too).