Just an idea that raiders won't necessarily like but would encourage gameplay and motivate to get money even on high levels.
How about you can't buy a raider forever but only buy 500 lives for 50k, everytime you die ingame or loose a match, you loose one life.
If you die very little, you have plenty of cash for weapons, if you die a lot, you don't.
It would also encourage people to spend more real life money, and it would not be pay to win. It might however discourage people from helping newbies, so perhaps mentor matches would give +5 lives as reward.