I can't give you any advice as to what it would cost to source and machine those parts, but I do think it's going to be more expensive than you think. My local machine shop charges $60/hour and it adds up fast.
You'll need precision plate (expensive) or start with extruded parts and have them flycut/faced to accurate dimensions (time and labor). I also think you'll need the screws, end blocks, and rails on-hand in order to do accurate drawings and improve your chances of success. I do think that 1/2" plate is reasonable for that size machine and NEMA 23 motors. No problem there.
Motors, wire, enclosure, connectors, breakout board, drivers, power supply can easily run $300-400 and that's for a pretty basic setup. Limit switches, Estop, relay controls - all extra money. Small expenses, but they add up very quickly. It all depends on how much you can scrounge and what the parameters are.
You might want to consider doing the construction yourself. If you have a chopsaw/tablesaw and a drill press, you can fabricate most/all of the parts yourself. If you have any machine shop experience, you can do the work accurately enough and perhaps save money instead of sourcing it out. 8020 extrusions are another option, with the compatible rails, screws from www.cncrouterparts.com
Yet another option is to pickup one of Widgetmaster's beautiful little CNC machines, which are ready for motors and electronics.
I'd encourage you to design/build your own, if you're looking forward to a project. If you're resourceful enough you may be able do it for under $1000. If you're intent on ballscrews and precision linear rails, that target may be very hard to hit.