These servos are cheap and powerful
http://www.servocity.com/html/spg785a-5_0_servo_gearbox.html
Attached is is picture with the scale of lift force of the servo.
The downside to them is they are bulky and require shielding around the aluminum gear to prevent damage. Fully geared down they are able to life a 6 cans of cola at 18" If you linkage them into a 4 bar mechanism you could easily double that.
On the Sketch Up 3d warehouse I loaded a 3d file to help with servo design in mechanisms. It is found under the name "Servo Tool kit"
It shows many different servos, their cost, their torque (lifting cans of cola at a certain arm length), and their type.
http://sketchup.google.com/3dwarehouse/details?mid=bd8897b1471e2d1c73ac8f193878e4bc&prevstart=0
-pb
