alexs, your job sounds identical to mine about 5 years ago and I was making about $30k and was in the central part of the country - a plains state.
Being fed up with the pay and knowing/learning everything I did, I asked for a raise and if it didn't come through I would look for another job. Well, I ended up looking for another job and ended up a contract AIX admin for a Fortune 400 company for 6 months the went direct to hire. My salary was $70k and it topped out for Senior Admins at about $90k. My contract rate was $25/hr. and then got $15k more when I converted. This was for a Fortune 400 company. I recently saw an ad for a Data Warehouser at a Fortune 100 company in my city and it was $75k/year.
When I was let go I applied for a job in the Chicago area for an SP administrator position, and when they asked by salary I gave them a range of $110k-$120k/year (based on the cost of living where I am and there) but they didn't want to pay that so I looked elsewhere. But a RS/6000 SP admin is a step up on an AIX/Unix admin because of the specialized hardware, software, etc., so the pay is generally higher($20k/year +) than just a regular admin on standalone machines.
The thing is, it is entirely dependent on location (cost of living), market, skills, number of machines, experience, etc. Specialists make more than generalists, too.
I have looked around on dice.com recently and have seen some AIX jobs in Dallas and Chicago that are paying $130k and $175k repectively, however, these are contract jobs, and specialized (AIX, SP, Regetta). And contractors generally make more, but since they bounce job to job every few months, or if lucky 1 time a year or if really luck a 2 year contract.