The USGS is having a huge sale on maps and other items. Sale ends May 7. http://store.usgs.gov/b2c_usgs/usgs/zIn ... chStart.do
This is a good sale if you want topo maps. They seem to be selling for only $1.00 each (plus some shipping and handling). This may be because they are phasing out the paper maps, so this could be a last-chance offer.However, all the USGS topo maps (including the old 15-minute maps) are now available free on the Web as downloadable PDF files. They are scanned versions of the maps, but they are very good quality. Go to http://www.nationalmap.gov for more information.