Wow, this is pretty awesome. I'm either a total noober and this has been around a long time and I just never heard of it, or this is a pretty new technology.
Here is a D-Link router -
D-Link DI-LB604 4-Port Load Balancing Router - with two WAN ports that does two things. It shares the internet connection to literally combine the available bandwidth. It also has connection failure response, which means, if one line fails, the other line will take over automatically to keep your connection up. I researched prices, that particular D-Link router is in the 100 dollar range.
If you want to do some more research on it, this technology is called Load Balancing. I did a search for "routers with 2 WAN ports" and came up with a fair number of links.
You really do learn something new everyday...