Bought only two 2x4s to complete this project. Ripped and chopped and milled like crazy to get it down to the dimensions of the plans. The pine is a little flimsy, I wouldn't recommend it unless you're on an extreme budget, but it CAN be done! Just know that any of the cross-bars that have a knot-hole will probably snap sooner than later. (Luckily I milled extras!)
Another cost tip is to skip the backing. I used two supports from the main material instead of any plywood. Watch out for wracking when doing the glue up and it'll be just fine. Fully supported with less weight too!