There are only a few mapped trails online. Trailforks.com has the most and there are also a few on MTBproject.com . There are several tour companys, Icebike and Icelandbikefarm come to mind, but your right they are expensive. You should be able to get to most of the trails in your rental car. Im going back in september and Im flying my own bike. That will be cheaper than renting from a shop while your there. Iceland is a magical place, enjoy!