Well, Sport cars usually do cost more in insurance. (because it is more prone to getting stolen, and the insurance people assume you'll probably drive reckless in it). I don't think anyone would be able to give you an actual estimate because there's a lot of factors that play into the pricing as...
who cares. just keep your words and pay her in increments. make sure the payments are in check so that you will at least have evidence. also, keep record of your email/message exchanges so that if she DOES take you to court you could at least say that you DID offer to pay and you WERE making the...