The 1 1/2" auger bit I got about a year ago cost me $75 so a bigger bit costing around a $100 sounds probably right. When I got my antique boring machine and bits, one of them is a 2" bit. I use this to bore out a 2" mortise. Mortises should be deeper than the length of the tenon to 'allow' for shrinkage of the timber that has the mortise in it. So clean 'bottoms' shouldn't be a big concern, cleaning out with a chisel usually does the trick. Mortises should be at least 1/8" to 1/4" deeper than length of tenon, maybe even more. Good luck, Jim.