The first thing to do is run a thread chaser down the hole to see if that helps.
You should be able to find that heli-coil kit for about half that price, try industrial suppliers. You will be better off if you use something like a Timecert, solid inserts work better than coiled inserts like helicoils. Princess Auto sells a plug hole repair kit with solid inserts.
Most shops can do this repair for you for less than $170 IF you don't mind doing the repair without removing the head.
When I had this problem the cost of putting on a new head was a little more than having a helicoil put in (labour costs to get at the engine). I couldn't find anyone that would do a timesert, only helicoils. I didn't feel comfortable relying on a spring to hold a spark plug against the pressures of the explosion of gas in cylinder would create and be pointing at my crotch at the same time. That's a whole level of kink I don't wish to explore.
Hmmmm didnt I do that head for you? That was years ago, VFR if I remember?
A helicoiled thread is stronger than the original thread for pull out strength on an aluminum head.