Man that really sucks, how many hours did they assist you, if weather was clear and you were not in any danger, the fee should be $125-$150 an hour from the time they left their dock. If they did not tell you about the charges before hand maybe you can fight it. I would look for an attorney with marine knowledge.
Next, check out BoatUS.com, for a small fee you can have free towing, I also have trailer assist.
Good luck.