The belt's price is not the problem. It's cheap. Not sure why, given that Deere's parts are generally obscenely expensive. Maybe sellers just want to clear out NOS belts for a 31-year-old machine. It's apparently an ordinary plastic and rubber belt, not something fancy like Kevlar, so it was not expensive to make.
There are two problems. One is that after spending half a day taking the tractor apart, I have to wait for a package or run a one-hour errand and burn about $9 worth of gas. The second problem is that the tractor has other belts I may need to change eventually.
Tractor Supply's cross-referencing is not to blame here. Their site doesn't offer cross-referencing. I didn't find any useful reference when I went to the store. I have to go in and buy by size.
I think it was a business decision because I've had many vehicles, and their alternator belts held up fine in spite of having a different profile from John Deere belts. I can only recall having one other v belt fail during my long life. I don't think John Deere has discovered the secret to belt longevity. If 35-degree belts work in cars that rack up six figures in mileage, they ought to work in lawnmowers that run two hours a week.
Maybe I'm wrong, but I can't help being suspicious of a company that spends millions fighting Right to Repair and seriously expects people to spend $250 for a lawnmower muffler. They also make sealed hydraulic cylinders that can't be repaired without machine tools and welding. No excuse for that.
I paid nearly nothing for this thing, and I don't want to have to spring for another diesel mower, but it seems like something goes wrong with it about one in five times I try to use it. It was built to be sold, not worked on, so it has been a source of frustration.