Not sure if there is a Shadow World specific answer, but Castles & Ruins gives a wage for an average guard of 3 sp per month, and 4-6 sp per month for a soldier. The PCs may prove themselves above-average of course. Some other relevant wages include constable 3-7 gp/month, chaplain 1-6 gp/month, huntmaster 1-3 gp/month, magic-user 1-10 gp/month, PCs might qualify for something like those positions depending on their talents. (Conversely, a cook will cost 1 sp per month, stable hands 8 bp, cleaning/maintenance 5 bp, servitor 5 sp.)
The outtakes for C&R give some example rents for fortresses. Barracks (1250 SF, 10' tall) 38 bp per year, small keep (2500 SF, 20' tall) 30 gp per year, small round tower (300 SF, 30' tall) 42 sp/year, large round tower (700 SF, 50' tall) 24 gp/year. Of course these would depend on the quality, location, and whether anyone else is interested in renting the structure. If they have to reclaim the location, that ought to be worth some kind of discount.