There are probably more farmers out here than you realize that have considerable electronics and computer experience that could probably build an autosteer system, but it all boils down to economics.  If I was going to develop something inexpensive with very little or no profit margin, it would have to appeal to the masses (i.e. hundreds of thousands or millions of users ) to make it at all profitable.  With high tech farming there just are not enough farmers in the world to make these items as inexpensive as a $400 UAV.  I've found the easiest way for me to save money is to buy used and use my tech knowledge to be my own support.  I realize that is also risky, but works well for me.