The 2" is $250 US so not sure where you are getting $300 from. This is, of course, via Biota direct which is all I would recommend anyway. Currently owning several, large, fish from Biota the growth rate is such that it would be a waste of money in my opinion to pay for the extra 1/2 ". I believe they did this because some complained about the coloration of the smaller size. No other reason. My most recent fish from them, Milletseed Butterflyfish, has already doubled in size in under 3 months. When fed properly young fish grow quickly.
If one wants a yellow tang that is the going rate to obtain one via proper legal means that isn't doing a hobbyist to hobbyist sale pre ban animal. Take it or leave it.
Edit: $300 number maybe included shipping which is a valid point. I personally don't factor that in as it is what it is. I just stuff the box with more stuff to make it worth it. Having said that shipping would bump it over more than likely to the $330 range depending on local.