I imagine there must be some combination of
Cisco router hardware
that would support dual WAN access (both cable modem and DSL) for a
home office, and there's all sorts of used Cisco equipment on Ebay.
But, Cisco's product line is utterly cryptic, and I haven't the
slightest idea how to figure out what, if any, Cisco hardware would
let you do a dual WAN setup. So no idea what it might cost either.