My longer answer...
At a minimum you'll want non-owned CFI insurance, probably with at least some minimal hull damage coverage to take care of an "oops" moment. Budget around $250-500 annually for this, depending upon the coverage you want. This covers you when you instruct in airplanes you dont own. The best coverage I've found is from Avemco through NAFI. This covers you but leaves the aircraft owner exposed to liability issues.
A limited, CFI-owned insurance policy will cover about 4 students at a time for around $2,000-$2,500 annually for a basic low-end Cessna/Cherokee. If you have more than a handful of active students, figure on paying $3,000-4,000 annually for a regular instruction/rental insurance on the same airplane. If you're clearing $30-50/hour above fuel, oil, maintenance, and reserves on the aircraft, realize that's about 60-100 hours per year of rental just to pay the insurance man.
You could try the route of having the students all buy renters insurance (not a bad idea anyway since the above policies will generally subrogate [pay you and then sue a student if they break the plane])... but realize that isn't giving you or the airplane owner any liability protection so it might be pretty stupid depending on circumstances, and the majority of the above insurance expenses are on the liability side, not the hull coverage side.
Basically, whether you own or lease a plane for instruction purposes, you need to fly it a lot and keep up on maintenance to be successful. Have a good cash reserve and sell sell sell.
Hope that helps.