I Just Don't Know...

...what to say about this kind of stuff. I have a doctor threatening to send me to collections if I don't pay a bill that he can't really justify from an ER visit over 2 years ago. I wonder why? My insurance won't touch it because he was an out of network, on-call doc and he waited nearly two years to bill me. My only other recourse would appear to be legal action, which would probably cost more than the bill.