Is there any reason why the mechanic of paying money mid-turn has been used so sparingly? (only on Black Market and Storyteller)
I'm asking for a friend. He is into designing custom cards, and insists there is "considerable design space there". I'm less optimistic, but can't articulate why to him, other than that I personally don't care for it.
Originally Prosperity tried out paying $ as a sub-theme, although the specific cards didn't work out. Other sets avoided it to let Prosperity have it, although later I had Black Market and an Alchemy (large version) card that let you buy a card to hand (revived and then fixed as Artificer).
Also, originally there was no Buy phase, and buying meant being able to play Treasures. This is why Black Market has a messed-up phrasing that assumes you can play Treasures.
Black Market confuses some people, though again part of that is just a bad phrasing. Also you need extra text to allow Treasures to be played. After Black Market I decided I would just do "discard a card" or "discard a Treasure" in place of "pay $1," depending on how generous I was feeling.
"Pay $ to draw" started out in Prosperity; the card as I had it seemed crazy and I didn't try to fix it up. Stables does it, in a version that only lets you pay once. Then in Adventures I tried "discard Silvers for +2 Cards per," which seemed promising for a bit, and then somehow convinced myself to go for playing treasures and spending $ again.
So, it was initially a thing, but for the most part I personally find "discard some cards/Treasures" to be simpler and close enough.
Paying $ has tracking issues, but often you can turn the spent $ sideways. And there's doing it in the Buy phase.