I don't understand what you meant in your "Another note-" but as far as the way the situation was handled, NO, that is not enough time to expect people to fork out the money, and YES, that is too much for a shirt for preschoolers.
Personally, I think I'd do a couple of things. I'd see if you can buy a patch or something with the school logo on it, and make your own shirts.
If not, I think I'd buy one, and interchange it, maybe every other day (or when I got to washing it) with plain yellow shirts.
On the other hand, I'd also want to know how many of the other preschoolers will be wearing the uniforms. If most of them are going to continue wearing their regular clothes since the uniforms are optional, I don't think I'd even bother with them.