I'll throw out some tidbits. Single Use (SU) motors come preassembled. The propellant grains and O-rings are already in the liner and in the casing. The casing is disposable. The delay element is preset and the black powder (BP) charge is packed and loaded. All you do is insert the igniter and throw it away when you're done.
Reloadable motors require assembly. You buy the casing and closures one time. After that, you buy the reload kit, which contains the grains, the delay element (which is adjustable), the O-rings, and sometimes the nozzle (some manufacturers include a reusable nozzle with the hardware).
With reloadable motors, you have the up front expense of buying the hardware. But the reloads are much cheaper than SU motors, and eventually you come out ahead on cost. Unless, of course, you lose the rocket with your motor hardware in it.
Whereas most SU motors ship with a HAZMAT fee, many reload kits do not, as long as the grains are small enough and separately contained. So if you are buying motors online, you also save big on shipping. However, this is only true of smaller smaller reloads.
Reloadable motors require the additional labor of having to clean the hardware after each flight. Although there are some SU motors in HPR impulse, most HPR motors are reloads, and thus advancing in the hobby will ultimately require that you know how to build a motor.
Hope this helps.
