We will probably never get consensus regarding questions on how much money one wants to work for? Mostly going to depends on country/region one is from and situation person is in.
3D modeling also has allot to do with experience, some are inexperienced and not know own value jet and work for less, while experienced workers look for better paying jobs that demand experienced workers, etc.
With this in mind I would say just figure out how much you are willing to spend and try find the person willing to deliver “good enough” result for that amount (what type of artist do you want to attract is probably the main question here)?
Probably best post jobs per model (get them in FBX or OBJ format to avoid limiting to Blender artists only) and maybe work with a bit lower starting price in beginning but leave it open for negotiations with artists, see how much more they would need to get the work done.
Per model jobs gives you a chance to work with different artists and see what works best. Maybe some artists are willing to do a bit more if they get more jobs, etc.
If you need full copyright/ownership (do not allow the designers to license out/sell models to others) then its going to cost more in most cases, but jet again, it depends on artist your getting.
I would recommend going for regular license and leave ownership to the designer (unless you are delivering detailed concept sketches and all design is in fact yours).
