Cotton=BAD.
You essentially get what you pay for in an undersuit but here are a few tips to help with your purchasing decisions (from someone who has done their homework).
Rule #1 NEVER wear a cotton tee shirt, PERIOD.
Look at the label. Nylon or polyester is what you are looking for as those materials do not absorb as much water as cotton does. They dry much quicker which promotes the cooling sensation and does not leave you in a cold sweat when entering air conditioned indoors. Cotton absorbs water at an enormous rate, swelling up the fibers and closing the pores of the fabric. The fabric gets heavier and traps air between the skin and fabric, causing overheating and potential dehydration (unnecessary excess sweating).
Wallymart or any generic sports stores have them for an extremely low cost that perform better then a tee shirt, and that's a good start.
If your looking for the next step up, that generally means that you don't want to smell bad, deal with fit issues or chaffing, and you want better performance from the fabric that will do a decent job of keeping you cooler, lasting longer, and reducing dehydration. Again, look at the label. If you see Lycra, or Spandex along with Polyester or Nylon, you are getting a decent product with some stretch (more comfortable). If you see Elastine, that means the fabric is made of a cheaper no-name elastic which will degrade quite quickly, stretch out, and fall apart. That generally indicates the overall quality of the fabric and namebrand. If they are purchasing the cheapest fabric available then they are likely finding the cheapest sewing etc.
It is important to note that 2 different polyester or Nylon fabrics will perform very differently. The way in which a polyester or Nylon thread is woven together to make the fabric also affects the performance of the fabric (quick drying, better breathing). This is where the "you get what you pay for" concept kicks into high gear. There is a reason why cycling apparel is very expensive.
This is a piece of kit that is growing in popularity in this industry. At the track it is a necessary must. Its a good idea to go with a product that specializes in motorcycle baselayers because the fit will be specific to riding (and fitting under leathers) and your supporting the sport you love so much.
I start with Underarmor and moved my way up from there. Just being able to don my leathers easier was a massive deal.
Hope this helps guys!!!