Hmm...gotta watch what insulation you put in them. Gotta be high temp stuff. I got my insulation for about $150. I got 22 Gauge and 20 gauge sheet metal for mine. I got my control box made by an electrical engineer and it's digital with a timer. a little high tech, but at the time i could afford it.
All in all, I spent around $1,600 building mine. A little over budget but worked nicely. Just make sure your dimensions are good on the frame, because you don't want to be bumping it putting that frame in the oven after it's got powder on it. I know mine was still a little tight inside right at 3' wide, 3' deep, and a little over 5' tall. If you plan it out, then it will work nicely for you! What was you going to use for heat? Make sure if you use regular stove elements, it clears your workpiece with adequate space so you don't over cook the powder in some spots, and under cook it in others. You want a nice uniform heat put to the workpiece.