I would depend on how much work he/she is going to do for you. If the seller is not paying then I assume, since I don't know, it is a for sale by owner. If this is the case I would want the realtor do do all the work he would normally do if he were to list it. I would want to have an accurate information sheet outlining the sq. footage, room sizes, lot size etc. I would want him to pull title, I would also want him to get all the sold compatibles of like properties for the past 90 days etc. I won't want to trust the owners information. to bias.
On top of this I would want him to write up my offer and present it to the seller and negotiate the best price for me based on the market evaluation (get all paper work done)
Since the realtor did not have to spend any money getting the client or showing homes to you (which is very time consuming) you should be able to negotiate a reasonable fee. I would just ask the realtor what he would want for his service. Just keep in mind the cheapest is not always the best. Get a realtor with knowledge. Its worth it. Also remember the realtor could be showing a customer that pays full fee instead of working with you. Time is money for a realtor since they work on commission.
Real Estate Investor