There is no cheap solution that will keep track of usage and switch between. Even a dual home router you have to manually change the traffic.
If you only have a single machine the "easy?" way to do this is to run a bandwidth tracking program and then adjust the routes manually.
The key to doing all this is the ROUTE command in the PC. What you do is place both your router in the same network say 192.168.1.1 and 192.168.1.2. In your PC your would in the simplest case change the 0.0.0.0 route to go to the device you wanted to use. If you are more adventurous you could send some IP to one and other to the other...of course you would have to spend the time to figure out what sites use what ips.