It may be a fair price now, they used to go for double-triple that.
Other CBs can be made to work, but not with the Interstate handlebar controller.
They were designed to fit underneath the trunk. There may be a few other mount parts needed to make that work (or home made parts).
It'd probably be nice to have a Valk CB instruction manual/pamphlet. The handle bar controller is not exactly intuitive. It's on here (if the link still works)
http://www.valkyrienorway.com/download.html