yes worth the $$$. the UA shirts are nice and will last a long time if you follow the washing instructions. I buy UA loose shirts and the nike copy ones. The nike's are cheaper and fit is also nice. I also buy the UA boxer jocks.
cheap cotton shirts are great to, but if you're doing cardio, the retain sweat like a mofo. But, you can always just pick up some black or grey hanes undershirts, the cotton is thinner so they wick sweat away better then a regular t-shirt.