If you're not making money on combat now you've not mastered the system. There's also strategic considerations so you do ensure your booty surpasses your ammo (and merc salary) costs.
Don't forget stim costs. Duraplate is expensive, but you'll see a HUGE reduction in how many stims you need to use after each combat if everyone is wearing duraplate compared to anything else. You can easily go from needing 15+ stims after a single combat if everyone is wearing leather to needing at most 1 or 2 stims for the same fight wearing duraplate. That adds up fast.
Once my main gets some of the health and healing implants some days I don't use stims at all during a day of fighting and just rely on free heals from the girls and from sleeping, saving the stims for the mercs.
Ammo is a big one though if you're not using the top tiers. You don't really want to be paying for anything more expensive than rifle ammo unless it's a tough fight.
That highwaymen encounter is a bit of a strange one for me. I usually find it unprofitable unless I have good luck on drops. With 5 enemies that all do high damage and also have decent health it's not possible to kill them all quickly so I usually take heavy damage even if everyone is wearing duraplate and using plasma guns. Usually they drop zero guns when I do it. I tend not to bother with it unless I'm already on my way to the ferry for some other reason.
Also there are some battles where it's better to take out 2 weaker opponents in round 1 and save the strongest for later if you can't kill that one in 1 round. Some of the strongest enemies have so much health that you can't kill it in 1 round even with 5 plasma guns and a 100% hit rate unless you get a crit or 2. So if you can take out a couple of assault rifle wielding opponents wearing leather or combat armor the first round then you can work on the big guy after that.
If you're set on always taking out the strongest first, you should pick up Ayden. He does by far the most damage of any merc in the first round despite not being able to use plasma. The other mercs can overtake him in later rounds but if you really need that guy with the plasma gun to never get a shot, Ayden's your man. He can consistently 1 shot someone wearing combat armor without needing a crit and can take someone in duraplate down to about half in 1 shot. Nobody else is even close for round 1.