How long have you been looking for properties, and how good is this compared to other options. This is how I would proceed.... basically the same as @Sean Carroll said.
If it looks like this is the best deal going, get the purchase closed, and shop for better insurance. The key to real estate is time, so the sooner you get in, the better... you might loose a little money when you're living there, but that's better than paying rent, and as long as it cash-flows when you move out, great. You can always sell, refi money out, or something else once become more experienced.
However, if you haven't been looking very long, or there is nothing special about this property, and you are not emotionally attached to it, make the business decision. Tell them you haven't found insurance close to what they stated and cancel the purchase agreement.... and do it quickly to do both of you a favor. You can say something like this... "I'm sorry, but With the insurance quote I've got, I'd only be able to pay 90 (or whatever)." That way, you are not being confrontational, but leaving things open should they want to come down in the near future. I've had this tactic work for me in the past... though I was just being honest, not trying to negotiate.