It seems most every stager has his/her own business model. Most will come and do an initial report/evaluation/assessment, often in the $200-$300 range. Then it varies, depending on: (1) whether you simply want to take the report and follow through on the recommendations (inexpensive, but not recommended); (2) whether the stager him/herself will do it, or hire someone else to move furniture around; and (3) whether you need any furniture or props to be provided, and whether, if you do, whether the stager has them in inventory or has to rent or buy them.
Overall, you could be looking at several thousand dollars. (Larger homes and vacant homes will cost more; smaller homes and ones with items the stager can work with will cost less.)
At this point, you probably don't need to hire an assessor. Interview three Realtors. Ask each for a CMA, look at the comps they used, and discuss that with them. You'll get a very good idea of the range in which your home will sell.
Hope that helps.