Your Realtor IS the best person to ask! There is no good formula for how much to offer compared to list price. It depends on how accurately, competitively, aggressively the house was priced to begin with. Your Realtor can check comparative sales in that neighborhood and/or for that type of house. He/she can check the history of the property for prior sales records. Keep in mind, if you are asking the sellers to pay your closing costs, that comes right off the top of their proceeds. The sellers will be paying the Realtors' commissions as well. Talk to your Realtor, he/she is the professional!