Even going the SPAD route, a good engine will cost at least $50 and a decent 4-channel radio outfit will be another $120 or so, if you get them used (check the Swap fuorums). Add another $40 for wheels, fuel tank, pushrods and other miscellaneous but very necessary hardware, and $35-40 for fuel and field equipment.
Minimum total investment - $250 give or take.
The good news is that once you clear the initial hurdle, most stuff is recyclable into your next airplane, and the next, and the next.