Tp-Link have some Load balancers/multi-wan routers that are very reasonably priced, but I've not tried them so don't know what the limits are in terms of throughput.
They're Wired only but it wouldn't cost much to get a wireless router and set it up to just be an access point.
http://uk.tp-link.com/products/?categoryid=227
RouterOS can also handle multiple Wan connections but it's a bit of a pain to setup, the lower end Routerboards might have trouble with the speed, My RB751 tends to top-out at 40Mbit/s when doing Nth packet routing down L2TP tunnels
Not sure how much of that was the L2TP overhead though.