Now a lot of people here probably lurk around BGG and may have seen mention of this before, but just in case (I only came across it today by accident) here are a couple of links to threads on BGG about using a service on www.artscow.com to print playing cards with custom designs both front and back. I'm still looking it all over myself but it seems you can get a custom deck of 52 cards for about $5.00 (about three quid to us over in the UK), which I'm sure you'll agree sounds pretty good. Anyway here are the links to the most relevant threads on BGG:
www.boardgamegeek.com/thread/395699