Looking for something a bit cheaper and smaller. I'm able to buy a 2.1 system from Edifier with a 4" sub for about $40. 

I'm not that much of an audiophile to tbh. I currently have Logitech Z150 speakers and I'm quite content with them. However, my friend recently got a 2.1 system for his PC and now I just need a subwoofer. So what subwoofer should I get? ONLY a subwoofer, I want to continue using the speakers I...