Good question.
As far as the FSM staff goes, we're really journalists/editors first, and modelers second. We really don't have any full-time modelers (who just model) on staff.
I know a couple of people who build models for a living, but they do scratchbuilt, museum-quality pieces on commission, not kit-based work. Most were special-effects modelers in the era before CGI, and are used to building complex scratchbuilt pieces on a tight deadline.
We pay our contributors pretty well, but I imagine most of the guys look at the payment as extra money as above-and-beyond their regular "day job" salary. Money to spend on more kits, I'll bet!
Matt @ FSM