I'm wondering if you have an attorney handling this matter for you, or are you just dealing with the realtors.  Usually the contract for purchase of the house would address mediation issues, including who pays.  If that is not the case, prepare a formal letter, or ask your attorney or your realtor to do it, setting out the conditions under which you will go to mediation, including that the buyer pays for the mediation. You may wind up having to negotiate that point and perhaps having to split the cost, but start out by requesting they pay it.