The price of a transmission build can vary widely depending upon how its quoted. Some builders will follow a more conservative approach and reuse certain components that others following a more CYA approach would scrap and replace regardless. The low bid may be based upon all normal non consumables are good for reuse and then they start nickel and diming you as they find parts need to be replaced, vs the high bid where all the bushings, pump gears, new front planet ect are all automatically replaced. A detailed enough quote saying what exactly will be done/parts replaced would be the best way to know what your $ is buying.