The overall average for this is about $3,000/course, but I believe this is brought down by small private & for-profit schools. At a typical public school $4000/course is a good rule of thumb. Teaching more than 4 courses is a significant load, and some schools limit it to 3 or 4 courses/semester, but let's assume 4 courses in the spring, 4 in the fall, 2 in the summer. That puts the yearly gross at $40,000/year. That's probably a livable wage in many parts of the country, but keep in mind this does not include things like sick or vacation pay, health insurance, retirement plans, or other benefits.
EDIT: I should point out that the above is, for the most part, a best-case scenario for adjuncts. It often isn't possible to get full teaching loads, or doing so requires piecing together a course here or there at multiple schools. Income in the $25,000 range is not uncommon.