I’ve got this chinese 3040 CNC frame some time ago from RATTM MOTOR CNC on aliexpress, for just US $ 536.07, shipping included. This is not a ready-to-use machine; you should put your own chains, steppers, stepper mounts, controller, etc.
This frame looks pretty good to me; It comes almost fully assembled and every axis has a pretty good sliding action. Of course you should clean the whole thing and re-grease linear bearings and ballscrews before use. Please note I did choose the version with detachable spindle bracket, so I can install whatever spindle I wish.
I’ve just finished installing wire chains, steppers and tested the whole thing. Next step would be to make an adapter to mount my 12.000rpm DC spindle.