Well, the Ball screw cogs will depend on what size ball screws/ends you choose. The screws from ST are cold rolled and are only about 0.004-5/inch. You can get them cheaper elsewhere, but you will need to turn the ends yourself for your bearing types.
Dan Mauch sells the Clifton 140 oz/in servos (with you choice of encouder types from US Digital), for a good price that will run your ST very well.
Drives you can get from Gecko 320's if you want servos which are superior in my opion to the steppers in that same size range.
You can buy a Transformer, Caps, Bridges, bleeders, etc. from places like Mouser, allied, Jameco and others. (Including shielded signal cable, and other wiring from the above places as well). Dan Mauch and others also sell "Kits" that contain the above that you can assemble yourself.
Since you have a ST you can make your own mount plates.
Your probably looking at $1,500-2k for all the stuff you you need to do the macine. Don't forget the computer/OS, and other miscelanous stuff.