It's the food that gets me more than the upkeep, personally. Salary and the effects on not having it I can deal with a'ight, having a bunch of diseased crazies sans morale (due to starvation) choking up your everything is... less treatable. At least with undead you can just leave stuff sitting around until you need it without much direct consequence... r'yleh, you either move it or worse-than-lose* it.
Though with cultists, they should probably attack or something (form splinter cults, generating those stealthy preacher thing events?) when not paid instead of just leaving. Or much better, do that to provinces neighboring your nation, or perhaps just have a high chance to not be detrimental to r'yleh (instead of causing unrest or lowering dominion or whatev', it would give a small boost to PD** and possibly just flat out act as an H1 priest, with some knock-on events). That, actually, would be amazing, taking one of the yokes around their neck and turning it into a direct advantage, and I think introducing another mechanic (the other being the insanity) that no other freespawn nation has.
... honestly, having actually spent some time thinking about it, now, it feels like the cultists really have just a massive amount of potential to be spiced up. Some of it would even make interesting mechanics for other nations -- having some that react differently to disbanding due to lack of pay would be both neat and rather incredibly thematic, particularly if there was a way to induce it that didn't involve deep sixing your economy. Events and magic sites that tailor themselves to the number and type of certain units also has a hell of a lot of potential... I actually want to say there's already a little of the former, but it'd be great if there was both more and more impactful/reliably activated ones.
*Which is half the reason I love BVCs and whatnot so much, the whole "here, have a unit you still have to pay for but now doesn't even work well as chaff" thing, with starvation switching that to "but now cripples anything non-mindless in your army", due to routing considerations. It just sucks to be on the receiving end :V
**Actually... if it's possible to remove magic sites, or have another building sorta' like a castle, it could act like those sites that add new PD types to the province, preferably with some kind of stacking mechanic so the more cults you have established in a single province, the larger and weirder the adds get. With losing PD or having the cults dug out (they'd still presumably be stealth armies loitering around, or something like story events you can search for) causing the PD to reduce... or having it so they stop being extra PD if someone else has control and starts being an overt nuisance.