I'm not a big fan of the C9 shirts at Target. The lower priced C9 shirts were very flimsy, you could see right through the fabric. The better made C9 shirts were too expensive, costing over $20. In the mid $20 price range I can find a good quality shirt from TW's discount section or go to Marshalls and find something decent. However, I understand it's not always easy to find discounted shirts if you are a size Medium.
A shirt that fit's well, is made of comfortable material, that doesn't come apart after a few washes and looks decent is worth a bit of extra money. But I only play once or twice a week and don't need a closet full of tennis gear.