ESOP valuation specialists charge widely varying amounts. Part of the difference is the firm's reputation. If your valuation is ever challenged, that reputation may be worth paying for, even if the work performed is the same as another firm with a lesser reputation. Another part of the fee difference is how many people at the firm review the appraisal. More reviews may mean more accuracy.
Appraisers also vary in terms of their philosophy towards ESOPs.
Appraisers also vary in the emphasis they place on different valuation methods, such as discounted earnings or comparable companies. You need to tell prospective appraisers what you want to do with your plan, then find out what basic assumptions your appraiser will bring to this potential transaction. There is not a single right or wrong way to arrive at a value, but the different approaches can result in values more or less appropriate to your needs and opportunities. Finally, you should request a statement from your appraiser indicating if the firm has ever been challenged on an appraisal and what the results were. The challenge may have been unfair to the company, of course, but you should know about it.