Note: this is copied mainly verbatim from one of my previous posts on /r/KerbalSpaceProgram. Just wanted to mirror some of my thoughts here.
Ore is $0.02 / unit. Monopropellant is 1.2$ / unit. (See here). 1 unit of ore can be converted to 2 units of monopropellant.
A converter converts half a unit of ore to 1 unit of monopropellant a second. Producing 1.19 profit a second.
Roughly speaking, it works out to ~25 704 profit / converter / (kerbin) day.
So: build a converter + power + full ore tanks + empty mono propellant tanks (2x the space for monoprop as ore). Convert ore to monopropellant. Recover vessel. Repeat.
This is much faster than the "standard" mining ore on Kerbin thing.