You will find the number of administrators for the commercial Unix variants (AIX, Solaris, HP-UX) is greater for Solaris and HP-UX than AIX, because Sun has the most machines installed worldwide. Solaris and HP-UX are more “Unix”-like and you will find similarities with BSD and/or Linux, unlike AIX.
IBM pSeries servers have more power than comparable Sun servers, but IBM has a larger price tag, too, which might be a reason there are not as many installed systems. However, when you get to large companies, like my previous company, which was a Fortune 400 company, they will have multiple vendors (they had AIX and Solaris) but will lean more toward one (they leaned toward AIX). It is even a more narrowly defined field for AIX if you include the SP2, which I have worked on for over 5 years. There are even fewer SP/PSSP administrators than AIX administrators.
The pay for an AIX administrator would be the same pay that a Solaris administrator makes, though I saw a survey once that had a Solaris admin making more than AIX (probably because there is more work in administering Solaris servers), but really it will be the same. Where you will make more with AIX is going the SP route, where there are fewer installed systems, but the complexity is greater and the systems cost a lot more. Also, the workload on SP systems are normally mission-critical and are not transparent to the company’s business, thus downtime equals large losses. One system at my former company (the Fortune 400 company) had losses of $1+ million per hour of downtime. For those types of systems companies will pay more for a knowledgeable and skilled administrator.
Bottom line – yes there are AIX admins who do make money, but they are more skilled (SP/Regattas, mission-critical, etc.) but they are fewer than Solaris/HP-UX and the money will be about the same (unless you go SP, etc.).