First, you need a real crossover for both the main speakers and the sub. Second, you need a level control for the sub. Third, you need something to power the sub.
It appears that you are very "price sensitive" so I must tell you that I know of no mainstream affordable products that will suit your needs, new or used.
What will work and is inexpensive is a plate amp from sources like:
http://www.parts-express.com/wizards/searchResults.cfm?srchExt=CAT&srchCat=505&CFID=5871862&CFTOKEN=96349160
http://www.mcmelectronics.com/search.aspx?C=&K=subwoofer%20amp
Kal
It appears that you are very "price sensitive" so I must tell you that I know of no mainstream affordable products that will suit your needs, new or used.
What will work and is inexpensive is a plate amp from sources like:
http://www.parts-express.com/wizards/searchResults.cfm?srchExt=CAT&srchCat=505&CFID=5871862&CFTOKEN=96349160
http://www.mcmelectronics.com/search.aspx?C=&K=subwoofer%20amp
Kal