u1 batteries for wheelchairs

The most obvious answer to the question “What is the cheapest rechargeable battery for wheelchair power?” is the ubiquitous u1. But there are other batteries, too, and the ones I use are the least expensive option, costing about $40 each.

That’s right, $40 a battery. But when I look at the price of the batteries, I get confused because the u1 batteries in particular are all about the same size (about the same weight), but the u1’s are less expensive.