I don't know about cheap, but two layers of 2 x 12 + 2 x 6 work well.
I had to do that for my bus because my driveway was and still is a swamp right now with all the rain we've been having. The boards are 8' long and the tires are centered over the 'platform' to distribute the weight. They were a pain in the butt to move, but I'm glad I used them!