As part of a mentorship/tutoring thing, I built a robot arm with a student participating in a competition. The design is my own save for the gripper which is modified from this gripper from Thingiverse. This was built mainly during weekends over this January 2016.
The servos are two HS-53 at $8 a pop, a Hitec HS-311 at $8 also, a Tower Pro Metal Gear 995 and a Tower Pro Metal Gear 996 (about $10 each).
The rest is hot glue, wooden poles, zip ties, tape, a paper tower roll, 3D printed brackets. etc. The shoulder bracket was borrowed from a biped robot kit, but could be printed.
The arm is controlled by a smaller model that has potentiometers embedded in it. An Arduino takes care of mapping the values to angle and reflecting it to the servos.
The counterweight is very necessary. This arm reaches nearly 75 cm outstretched. The shoulder (highest up servo) output interface is the biggest ongoing problem. The horn strips out after hard collisions or just slow wear and tear. If the servo output had bigger teeth that would be nice, or if I could find a properly sized hardened steel servo horn that would also help. But all I have is a slightly too big aluminum horn and a set of slightly too small plastic horns. And epoxy, and superglue.