Most employers provide health benefits for their employees. Both you and the employer contribute. I'm currently paying $90 for health and dental a month and it's well worth it. If you were in need of medical attention in the US without insureance the costs go up fairly quickly.
As a FI I presume (first mistake!) you would be covered on the schools policy. However, AOPA do provide Aviation insurance coverage through AIG. I have my pvt license and my insurance is $170 a year. I saw on there they have FI policies also.
If you can check out AOPA flight training magazine, usually a wealth of information on this kind of stuff on there. For $33 a year I got an annual subscription to the magazine, lots of resources on their website, and a Jeppesen flight planner application for my PC.
Where in the US are you thinking of instructing? It's currently a great market for instructors, my pvt instructor logged 100+ hours a month for 6 months and is now off to work at the regionals.