Ultrasound does seem to have some effect - principally, to create heat internally and thus promote blood flow - but I'm sure some of it is to give them something to fill up the 45 minute session with. All my physio really needs to do is give me another copy of the exercises and say "don't stop just because the symptoms have gone, you bozo", but I wouldn't feel I was getting my £30 worth then.
Most professional kit is expensive because it is powerful, as well as more robust. I suspect that a DIY ultrasound from Boots or wherever would be too weedy to make any difference; and if it isn't, I'd prefer to have someone more skilled than myself at the controls.
Unless used correctly ultrasound treatment can do a lot of damage and is not recommended for amateur use. However another system that uses the same principle is infrasound, which operate at a much lower frequency than ultrasound. The units have the appearence of a simple massager, and are safe and easy to use, and after using one for several years for arthritic joints, and often on stiff, painful muscles, the one I have is remarkably effective. The best known make is Nova Sonic, but I use the Boots clone, which is identical to the Nova Sonic but a lot less expensive to buy.