If you live near a university the library's geology section or specific geology library may be a realatively cheap source of topo maps. The libaray at my school has all the topo maps for the entire United States and a large format copier. So for the cost of copies you can get full size (albeit black and white) topo maps. If you live in or near a large city you almost certainly have a university or college near you (e.g. University of Houston). You may need to get a library card but even with that added cost, the maps should be cheaper then buying full size topo maps from the USGS.