'Camping' is not really a part of the game... we want players to actively participate in fights and stay strong/armed.
While I agree with you that camping is a big part of the game, whether Jim wants to acknowledge it or not... I think you've overestimated the amount of income you need to not "get destroyed".
At lvl 45 I'd worry more about leveling than upkeep. Just arm yourself with fairly cheap stuff and level up. As you level up, you will have your stamina restored to the max, so doing missions are really useful for your economy, as well as for your skill points.
Look at Bulletstorm, for example. She's in the gang "MONEY". 20k wins, 3k losses. She's lvl 160 and doesn't have 250 mil yet. That's when I'd start building my income. But before that, I wouldn't say it's really worth it. And camping before you have 100 mil at least is just mathematically wrong, since you gain more money from fighting and doing missions every hour.
//Formidable - DeathWish MGS