First, it would depend on what vehicle you plan on installing the system on. Bigger the engine, bigger the unit has to be.
Secondly, IMO, you won't find a system for less than $250 that's worth a damn. They'll run hot, not produce enough hydrogen to accomplish anything, and they won't withstand the rigors of highway travel. My small cell, which produces 1-3 lpm continuous duty, cost $200 to build and I won't sell them for less than $400.
As for those propane assist kits, check and make sure the propane tank is included in that price. I looked at them about 10 years ago when I had a diesel truck, and you had to buy the tanks separately. And they had to be DOT approved tanks or the outlets won't fill them, and at that time they cost more than the kit.
Sorry if I sound negative -- it's not my intent
Steve
1991 Plymouth Acclaim 3L V6.
1 dry cells with nineteen 6"x8" 316L ss plates, driven by constant current PWM set at 35 amps (13.3V at PWM). 28% KOH electrolyte. Total measured output 2.5 lpm. Mileage went from 18 to 26 mpg, all city driving (44% increase). EFIE set at .370 and I still need to play with ignition timing.