In my opinion, very low. I've not heard of any American ISPs closing someone's account because they use Tor.

As a bonus, if you use Tor, they won't be able to inject jacascript into pages you visit (I know for a fact Comcast does this), and the traffic flow data they may be selling will be less valuable to the buyer (orther than that they'll learn that Tor is getting more popular).

Now running a Tor exit node is a different matter. If your ISP gets complaints because someone is spamming or "hacking" from your house, they're going to send you a nastygram and threaten to cut off your access.

If you are just using Tor browser, you're nearly guaranteed to be fine.