I'll use myself as a proof by counterexample. When I was buying my t-shirts from places I actually wanted to support, like ThinkGeek, or buying solid colored t-shirts with no logos, I found that the poor fit of the t-shirts was making me look bad and feel uncomfortable.
Eventually I was talked into going "shopping" by a couple of female friends (actually, I talked them into being my guide), and found that the only t-shirts I could find that made me look and feel good, with a wide range of prices, also happened to have well-designed corporate logos on them.
So, until it becomes technically possible and socially acceptable to have custom-tailored t-shirts that fit perfectly, have the right fabric, pattern, and seam/collar/hem, and are emblazoned with my family crest or personal corporate logo, I will continue to buy whatever looks and fits the best within my price range.
ThinkGeek apparently uses Gildan shirts, which are cheap, baggy shirts made out of thick cotton; you were probably buying shirts of similar quality when you bought plain shirts.
Since it seems you're spending more on shirts now, you're probably getting a better quality product. Plenty of stores from Target to Gap to American Apparel and so on have better-fitting shirts made out of lighter-weight material with no visible logos, likewise in a range of prices.
Eventually I was talked into going "shopping" by a couple of female friends (actually, I talked them into being my guide), and found that the only t-shirts I could find that made me look and feel good, with a wide range of prices, also happened to have well-designed corporate logos on them.
So, until it becomes technically possible and socially acceptable to have custom-tailored t-shirts that fit perfectly, have the right fabric, pattern, and seam/collar/hem, and are emblazoned with my family crest or personal corporate logo, I will continue to buy whatever looks and fits the best within my price range.