The servos are certainly where you can spend most money when building a robot for research. I tried building small research robots with small servos ($25 each), see results at the end of this video: https://m.youtube.com/watch?v=q8jgu-EtCFc

The total cost for one robot was well below 1k. The servos are $25 each (Turnigy 306G if remember correctly), all connecting parts are 3d printed, the electronics and batteries are about $50, and you could add a raspberry pi for $50. So in total <$500, depending on number of DOFs.

Of course, these servos don't have the same torque as Dynamixles, for example. But they are lighter!