IMTT USA (
http://imttusa.com) has a line of very affordable brushless motors. These are not being marketed specifically as servo motors, but my tests show they work very well in precision servo positioning applications. They do NOT have an integral encoder, however, so you need to provide an encoder somewhere in the system. Probably the best option is to drive the leadscrew with a toothed belt and put an encoder on the end of the leadscrew with a flexible coupling. The smallest size 34 motor has 50 In-Oz continuous torque and lists for $89.70 in single quantity (they also have a sale on right now). The largest has 198 Oz-In continuous and is $149.29 list.
I have tested these motors with my brushless drive and they work well with very little hum or vibration. The only interfacing trick was to use 1K Ohm pull-up resistors on the Hall outputs.
I can supply connection details to anyone wanting to hook these motors to my drives.
Jon