This is just some tips of how to earn gold quicker at mid-level solo

1. Upgrade SMG and Minigun
2. Upgrade Salvage
3. Use SMG, Minigun and Salvage
4. Use 3-4 oathes on bronze setting (Earnt more on bronze with 4 oathes then on Silver with 3)
5. Try and kill mobs close to you can in the same area, Spam salvage after every 20 or so mob kills.
6. Figure out which map is easier and quicker for you to solo.

Remember to break all pots, either by running thru them or shooting them some have coins in them.
Fully upgraded salvage will drop 1 loot per corpse (not 100% sure), loot meaning either health, armor, ammo or coins.
Ideally you want to wait until mobs come to you and kill them all in the same area, creating a "loot pile" which you can then stand in and get the armor, health and ammo as well as picking up the coins easily.
I mainly use the minigun, once I use salvage then switch to SMG to kite mobs as killing them after the ability has finished to then restock the bar.
Also skeletons do make loot with salvage even thou they do not leave a visable corpse.
In a few runs I was making 130-160k a run.
Using the Plasma Pistol instead of the SMG is another option.
Please rank/rate this guide if this has helped in any way. Thanks for reading. Hope this helps some people get some upgrades etc.