Well, it actually is fairly linear.
1/8 to 3/16 is a 1.5 factor. Their pricing is 1.7 or a difference of 20%
The difference between the 3/16 and 1/4 are directly linear.
Actually their costs are directly opposite of what I would expect. Mills run and sell by the ton. The thicker the plate, the more tons that run thru the mill at one time based on the same ft/minute of production.
However, thin gage material runs on different machinery than thicker gages although all of these materials would run on the same line.
Stainless is a funny thing too. It all depends on who runs what. Each mill has it's own unique gage range that they run and they may be getting their material from different sources.
I wouldn't worry about it.