This thread may also be closely related to [thread=281677]this[/thread] thread.
I went to the shopping center during my lunchtime today and smiled when I seen the Adidas outlet store front and it reminded me of this thread. So I went to browse around.
I would went in have asked the "difference" question to anyone if they approached me and asked if they could help me. However due to the time of the year and the shopping mall where I am is a tourist attraction, they had their hands tied in the shoe department. It would have probably just been like this thread and theories why but only verbally instead of written.
Around where I live, its a rare sighting to see actual tennis apparel unless the store specializes in tennis. This also holds to "small market" hypothesis that someone mentioned in this thread or the thread I linked. The Adidas store's apparel was golf, soccer, a little bit of rugby, and of course the "training" apparel.
For the golf apparel, I could make a full blown thread about it. It seems in the last few years golf polos have changed, following the fitting trends but also the material. The material has more of those polyester wicking technologies that tennis, etc. has and are closer to tennis polos. If anything modern golf polos can also be used for tennis and have similar designs and seams.
For the training apparel, I use a lot of training apparel for tennis because that's whats more available to me. Of course I have some actual tennis shirts that indeed look better and also perhaps its a psychological thing that if you play tennis, you might as well own at least one "tennis shirt" just to put a number on the stat sheet....lol. I am sure that a lot of people browsing these forums own a lot of the training apparel as well.
As I looked at some of the rugby and soccer apparel, it was $45 compared to the training stuff which was mostly $25-30 confirming the OPs rough price percentages. The training apparel did look good, but the other soccer apparel colors and designs I admit did look a lot cooler before I checked the price. I could imagine that there will be some percentage of the sporting population would justify spending that extra money for that cooler looking shirt. The soccer or rugby shirt (I forget which it was and I am emphasizing) was a black with lime color and had something text going around the collar.
It seems that with the emergence of these golf polos, it would give a tennis player even less of a reason to buy expensive tennis apparel. You can choose a modern golf polo or training apparel instead. This could drive up the actual tennis apparel up even more or it may become obsolete.
The only problem then is the tennis shorts. I believe tennis shorts are still made different than other athletic shorts with the pocket size, having pockets, and the way they fit. It seems to me that tennis shorts are on sale less than the shirts because of even a smaller market.