Although i understand the misconception here a lot better than with Potions. Potions and coin is just apples to oranges. Debt on the other hand "feels" cheaper than coin. If i can afford to buy a card costing $4, i can also afford to buy a card costing 4 debt, but not vice versa. This, in a "common sense"-way, makes it easy to come to the conclusion debt was just "cheaper coins".
I hadn't thought about it, but that's actually true. The Alchemy rulebook even compares gaining potion cost cards with buying:
"when a player uses University to Gain an action card costing up to $5, the player may not gain a card with [P] in the cost. ...
It is just like Buying a card - if a player just has $5, he cannot buy a card with [P] in the cost."
Obviously this thinking doesn't work with debt cost.
It *sort of* does. If you think of "debt cost" as "how much debt you have to take" as opposed to what you have to "pay", then in order to purchase a card costing
, you must have the ability to take at least
. The sentence above, in Debt terms, still applies... if a player just has
, he cannot buy a card with
in the cost. But you don't have "just"
, you have
+ the capacity to take unlimited debt.