... if you've got the stats for it, sewer runs are with zero doubt the most efficient way to make money in GH1. After a bit you start regularly walking out of those things with 2 million plus, consistently, for usually fairly little time invested. You just need enough to be able to tangle with a clump of, like. Twenty something albino alligator tier enemies while under the effects of a bad mood >_> Want to say the base was around 25 in primary combat stats, and either something like a couple of plasma cannons or what I mention below? Something that either hits AoE or never has a turn you can't attack with it, from range and without ammo. And your actual skills in the 6-8 range. Something like that.
If you can't afford rf bazookas, though, you're almost certainly not ready to go into one of those things, or at least not ready to be raking it in from 'em. Probably best to stick with other things. Don't recall if arena fighting or chasing missions is more efficient than the other, though.
Incidentally, I'd probably recommend some of the ground level weapon fighting I've forgotten the name of. That and enough deathwings to always have one off cooldown (iirc, it's around seven or eight) make for a hella solid non-mech setup. Infinite functionally-zero CD ranged attacks, that you can still wear melee weapons or guns while using. Plus they poison, and you can stick another doodad on them and have them do even more. Beautifully nasty stuff.