The Bureau of Labor Statistics' Occupational Outlook Handbook is always helpful:
Occupational Outlook Handbook, 2006-07 Edition
I looked on there but didn't find much (they do have a bit on general interpreting, however). You probably will find other useful information there in the future, so you might want to keep that URL saved.
To answer your question, it's variable. It depends on where you teach. If you teach at a community college or you teach at the university level, you're probably not going to earn much money at all (unless you're tenured at the university, then you make nice money).
For an idea, let's say you teach at a community college. The average compensation for instructors at the local community college is about $500+ per credit hour. So if you teach four ASL courses, that should be about twelve credits. Twelve credits times $500 is $6,000 per semester. That's pretty low. If you teach three semesters per year AT that rate (you'd go nuts, especially in the Summer), you'll make $18,000 a year at the local community college. Plus most of these positions are weak when it comes to benefits. Medical? Dental? 401k? Good luck!
This is generally true even if you're non-tenured at the university level. But it really depends on where you teach, your academic credentials, and how long you've been teaching.
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For anyone who's thinking about teaching ASL, this is my advice: I'd gun for a PhD (deaf studies? asl linguistics?) and try to get tenure at some university. You'll be teaching and doing research, but you'll also make a lot more money. More important, though, you'll be protected career-wise by the school and surrounded by the subject you love. Try the bastions of deaf academia: RIT, CSUN, Gallaudet, San Diego State, University of New Mexico, and so forth. They might be more receptive to providing tenure to ASL-related instructors than other schools.
I'm not the best person for advice on this subject, though.