I've been browsing around, and have found some of the rectangular-type control servos that are pretty beefy.
Here's one: http://www.hobbyengineering.com/SectionM.html#CatMS
Scroll down to the "Extreme Duty Servo". It's rated at 375 oz/in. It costs $80.
Lets assume it comes with a crummy potentiometer position sensor which you have to have to hack anyway to get continuous movement, as well as file down a stop in the gear assembly. No big deal. You'd have to find a place to bolt your own optical encoder, but I'd be really suprised if that was hard. Ignore the control servo line, and use the encoder line and the DC wires as you would an expensive servo.
Why wouldn't this servo be acceptable for a desktop system if you restricted it to, say, half its rated torque? Is there some fundamental difference in these servos that makes them unacceptable?
Unless someone knows of a source of decent gearhead motors for $80. Please share.