There's probably farms and such around. Not to mention it trades with Klamath.
That said, I suspect the place is not all poverty. That slave trade must bring good money, even if Metzger and his goons are the ones seeing most of it. The real social issue is the fact everyone is on drugs.
Most people probably work for Metzger directly or indirectly.
Also a good portion of the population is likely raiders and other criminals. AFAIK if you kill the Slavers but don't kill/weaken the Mordinos, the Den becomes a haven for raiders. Den is probably where many criminals and gangs go to hide, cool down, get some R&R - well, the ones that don't go to New Reno anyway.