Hello,
To ONLY 3D print out those skeleton parts, how much will that cost? Without considering the motors, and other electronical components?
It depends of your printer technology and Poppy model. I assume that it is for Poppy Humanoid?
Open the STL files in your slicer, it usually estimates the required length of material. Then according to the cost and length of your wheels, deduce the price 
As for motors and other electronical components, is there any cheap replancement for those things?
Do you want to replace them by cheaper actuators or make your robot passive with fake motor joints?
For the first, have you looked at the existing discussions and proposed solutions?
There are several attempts of creating adapters for smaller motors and you may contact the makers who built them if they did not share their files. Look at this adapter for example and see his proposal to share the STL files. Also check out this recent post where a Russian guy built a Poppy Humanoid with LX-16A motors. And search for Poppyrate that used AX-12 and XL-320 motors. There are many ways to explore if you want to use cheaper motors…
For the latter, you may find 3D models of Dynamixel motors online so you can 3D-print them.