I'm using vinyl flooring remnants of the new generation of foam backed fiberglass stabilized lose lay products on my concrete floor and they act as a decent insulator. The 230v Hot One heater at top center of the picture has worked exceptional. The garage is only 790 s.f. but well insulated (R32 doors) and I never run the heater thermostat past 50% and that keeps the garage at 68*. Based on electrical rates it isn't costing more than $25 a month to heat it.
The heavier weight floors from Mannington or Tarkett's commercial rating flooring have held up well and you should be able to get rems for $1 s.f. or so.
That's the budget rout but it's effective.