Having just listed a 3.32 acre lot in Blaine I looked over recent sales and price per acre in the River Ranch area. Price per acre was varied and hard to pin down an average. Location plays an important role in pricing, but the few sales that have occurred were spread from 12K an acre to 19K an acre. I did see several properties priced higher but they were not selling and had been on the market for a very long time. Feel free to contact me and I will be happy to help you get a grasp on land in this area that is not in a Restricted subdivision if that suits your needs better.
Have a nice weekend.