If you already have a rating, say, your PPL, then the expense of adding Instrument, Commercial, CFI and CFII might be around $16 - $20k without a plane. I guess owning your own, you'd have to factor the cost of operation, but seems like it could be done more inexpensively. With that said, if you're planning to instruct on the side for a very long time, it would pay off, though maybe not as much as other side jobs with a lower barrier to entry. There's also a high time cost to maintaining your aircraft, going to the airport, meeting with new students, marketing yourself, etc. in which you're unpaid. The part-time CFIs and owner/operator instructors I've met who seem to do well are the ones who can differentiate themselves and stay away from the high-volume stuff. You want to train the people with money who are doing this for fun, not career pilots. In Austin, I met a CFI not long ago who gets $7k to do accelerated instrument ratings with Cirrus owners - this is the kind of thing you want to do. A lot of pilots who make money on the side have additional, previous experience that allows them to work as ferry or contract pilots as well as instruct. It might be good to start out thinking of this as a hobby that offers some reimbursement, and if you can eventually make money at it, awesome. I'd say you might even be better off working as a part-timer for a flight school without owning your own plane, but sounds like there might be a unique opportunity where you're living. There is a lot of demand right now and it wouldn't be too difficult to get going once you're set up.
Another consideration is the ever-tightening vice the FAA is putting on GA, requiring new, expensive equipment, inspections, and whatnot. People with money also want to learn on aircraft with more advanced avionics and autopilot rather than the old-school six-pack. If your plane doesn't already have this equipment, these things are extremely expensive.
