They say .50 price steps are possible in the 10-50 range so X.90 + .50.

mynameisvlad · on Dec 6, 2022

So the way I read it is as two separate systems:

Developers can go in 10c price steps from $0.29 to $9.99 (0.39, 0.49, 0.59, etc), or in 50c price steps from $0.49 to $49.99 (0.49, 0.99, 1.49, 1.99, etc), etc OR developers can use the 4 supported conventions for any price in those ranges, as long as you fit within the convention.

But you cannot start at a supported convention and then use the price steps.