There used to be several threads regarding rarity though I don't think they were tied to pricing - in any case prices have plummeted since these were set up. You could probably put together a pricing guide tied to these groupings within a few minutes, though.
I've noticed that since the S2 cubis came out, there has been some fluctuation in the S1 values. Some of the rare/semi-rare cubis seem to have gone down in value, while the ultra-rare ones like Rube and Rook have gone up somewhat.