If you can pick up an older Cisco SOHO class router, like the IIRC 1600 or 1700 class with the plug in cards for the appropriate Ethernet or DSL connections, they run a routing protocol that permits you to assign relative values to the various WAN connections, that can be used to accomplish what you want. It may be a bit more then you want to get into though to understand and do the programing of the configurations. I have seen some of these older routers at swap meets for $75 -$100, but you have to search for them. Also, the software that runs on them, is not officially included, as you are usually required to purchase it separately. It's wise to make sure the swap meet box has the software on it that you need.
I don't know of a home style unit that offers that, other then the fail over types mentioned by donheff.