Like anything, they are worth exactly what someone will pay.
Unless you have a stock of amps, all rebuilt, tested, and warrantied, its pretty hard to set a price for them. Especially considering they are built on someone else's design. Seems as though most of the rebuilds vary from amp to amp considering the original donor chasis. To be able to charge a specified price, you need to produce amps that are identical in looks, build, and sound quality. Without a company to take all the rebuilds under one roof, a varied price based on condition is the best you can ask for.