For my next game, we're going to focus on the Bastions and spend more time with downtime, time, and money and such. One thing I'm considering is switching the game to a Silver model, and having the coin conversions be 1/100. 100 CP equals 1 SP, 100 SP equals 1 GP, 100 GP equals 1 PP.
The beauty of this is all listed GP prices can be switched to SP without any math.
It doesn't change how anything works, but it does make it so CP and SP will get used for a bit longer.
As for below common rarity, I use "mundane" for that. Mundane goods are up to 10 gp, and something below Mundane is up to 1 GP, I just don't have a word for it. Common are "Common Magic Items", not common in general. There is overlap, though. Chainmail is a common item by pricing, splint is an uncommon item, and plate armor is a rare item.