For the singles, I'm thinking something like 15 cents per plus shipping. Wouldn't want to sell just four and be left with a few others though. Only six are pictured but I actually found another so the set includes seven total.
I also found these two other UMD cases. The left one is like a neoprene style sleeve that holds two UMDs side by side, and the one on the right is a hard plastic clamshell that stores up to three UMDs stacked inside. 50 cents each plus shipping for those ones.image