I built mine for about $40 using the BBD guidelines. Steel is getting crazy expensive. I think a 20' stick of 2x3 .125 cost me $66 a few months ago. Not long ago the same stick cost about $30.
Keep in mind that some say you don't have to cut the floorpan for the BBD connectors. On my 70 Dart, I would have needed to cut the pan which was unacceptable. I got them in using a rather crude technique which I'm not happy with at all. Sucks when you are pressed for time and can't engineer things the way you'd like. If you have torque boxes, that throws another monkey wrench in the plan.
If I was going to do them again, and I will, I'd step the bars at the rear footwells. If you look at the Mancini bars, that's why they have the long side plates that attach to the rear rails. This is to keep them from intruding into the rear floorpan area.