This is probably as cheap as building one. They have an optional back rest (this one has it) and can be fitted with all sorts of extras. It works great for my friends disabled daughter. The full chain case is nice for a disabled rider. She has a lot of serious balance problems. She can't hold on with two hands or keep her feet in place without straps. The front bar folds.
If you start buying parts at retail this will probably be cheaper.
http://www.pedalcarsandretro.com/Cab...520.html#image