I would just like to mention that although the idea of losing points for making piece high rarity may appear arbitrary, the most likely intention here is that higher rarity units are more complex on average.

Rarity and complexity are not completely bound to each other, but they have correlation because a new player has a big enough learning curve in front of them already to not be bombarded with all the most complex effects at once, and yet they still should be exposed to at least a few to know they exist. So even though you can label literally anything as common in piecemaker, ideally the unit looks similar to what you might find at that rarity in the game. This also correlates to themes such as mythological creature vs peasant, but that's another topic entirely.

anyway, the actual rarities are:
Common/Rare/Epic/Legendary

You will lose 0/0/5/10 points if the unit is
common/uncommon/rare/legendary.

lol