5 mana is still a lot to ask for the privilege of playing a second copy of Cabal Coffers in your deck, especially when you have to wait a turn in order to use it, but the amount of mana you can get if it survives will fuel some very explosive spells and turns
This effect can't reduce the amount of mana this spell costs by more than
Not only do they suffer from having huge diminishing returns, but the decks are so focused these days (especially in formats with large card pools, like Modern or Legacy) that its nearly impossible to deny them of a single resource
Don't play City of Traitors, pain migh hurt but having to sac the land somewhat defeats the point of a ramp deck
It doesn't have an impact yet on the B&R list, because at this point, Type 1 (Vintage) and Type 2 share the same list
(2025-06-06) In the Commander variant, a double-faced card's color identity is determined by the mana costs and mana symbols in the rules text of both faces combined