First of all the 3.4 is a non-interference engine. If the timing belt snaps on it the valves will not hit the piston. You can check by taking off the inspection cover on the LH side (from the front) of the cam cover. If it cranks but the cams don't turn that is your problem.
I had one done on my Olds and it cost around 500 bucks.
More info on this problem can be found at:
www.60degreev6.com These guys are the 3.4 experts!
www.W-body.com