Custom maps, scripted HQ, script support for normal groups/individuals. Cash as well but i see no reason, its easy to farm.
What i wouldnt want others to be able to get by pay is anything others cant get with their own effort.(eg official groups can get hq/script support/map requests. People who farm cash get cash).
Then, celso's idea to pay real cash for nrg/infernus. I would agree on this but only if players could buy these too but for way higher amount (eg 500k nrg 1m infernus, 2m for heli, 5m for shamal..)