Bidding stumps by the inch is a good way to lose your shirt. First of all, a 4" stump has 12.56 square inches, a 40" incher has 1256. 10 times the diameter 100 times the area. Plus, a larger stump will have more root flare, roots and depth to consider. All these things must be considered, plus specified depth, access, etc.
I used to bid all my stumps for my guy, who has a huge Vermeer 1102, and a 252. I try to get him $100-130 per hour. Any less of a figure is selling yourself short. A stump grinder is a very high maintenance item. Now I borrow ( do trades) or rent a buddy's 252 for small or backyard stumps, and try to get $100 an hour or more.
Just did an alder for $200, 1.25 hrs shop to shop. A little high, maybe, but it was a rush job.
Obviously, if you have to compete with lowballers, getting good prices is tough. The only liability with stumpers is rock throwing (use hinged plywood) or hitting underground utilities (use a locator service), so cheap competition comes out of the woodwork......
....as if it didn't with tree work.....