Started originally focused as a mercenary/combat build and had two slaves with rifles (michelle/juno), Peasant w/ rifle, and Loren w/rifle. Cost less than $100 a day, not including using some stimpacks after some fights.
I don't remember it being too hard to regularly cycle through Watery Eye's freebooter, Celior mercs, Marston Ave mercs, and Stokke hill bandits, etc. You should be able to collect enough rifles to give you extra 30-06 ammo to keep this going for awhile. You can further supplement this with arena fights. I rarely bothered with fights outside the city.
With the latest updates, when you get good enough armor across your entire party to tank weaker enemies you can start dedicating at least one character with a combat staff to almost guarantee a male slave every fight. These guys can be sold for about $300-700 extra per fight.
Now that this run is about 500+ days in, I abuse the ammo pack mercs when I feel like doing any combat:
Aria (free) or Enya ($45) w/ Power Katana (Loren would be better /w +3 free dmg, but my Enya has about 139 melee from using her so long)
Space Marine ($260) w/ Plasma Rifle (ammo pack)
Connor ($330) w/ Plasma Rifle (ammo pack)
Mr. Doyle ($25) w/ coil gun
~$660 a day which is about one arena fight if you don't have any investments or other income streams.
Oh, also don't forget bounties. I don't think it's possible to take the actual bounty as a slave, but at 4/6 up you can start getting coilguns and plasma rifles for basically free after almost every fight (6/6 fights can rarely give two plasma guns, which is like an extra $9k). Plus whatever loot money and the bounty reward. Just remember to wrap everything up before 21:00, so you can turn in the bounty for a fresh one the next day.
Bounties will be important later when your control starts reducing the amount of bandits being respawned.