Having been through the used motor market myself a year ago, I never could find a decent reference for prices on used engines. Condition is going to play the deciding factor on how much you are willing to pay. You need to have them checked out by a Yamaha Mechanic including compression, leakdown, etc. and plan on going through them to replace fluids, water pumps, filters, plugs, etc. See if you can get this thrown in during purchase and then you have a basis for deciding how much. Also, a download of the engines CPU will allow you to review operating conditions (I believe) and look for potential problems. You will be able to verify the hrs on the blocks at that point as well.
Most importantly, if you are shopping for your Kingfish(er), and it is the 25 ft hull with a bracket or transom notch, these engines (IMO) will be too heavy and too wide to mount.