The main difficulty you're having in using the formula is that there is no existing spell that models the effect you want. Both Major Creation and Fabricate are dramatically more powerful than the effect you want in that they can create almost anything. Placing the limitation of converting coins of one type into another type of precisely equal value is a huge limiting factor.
For my part, I would put "convert currency" at a pretty low spell level (0 - 2). This is because I don't typically play campaigns where encumbrance or money changing are relevant... If either of these mechanics ARE relevant to your campaign, adjust accordingly.
Based on that, you'd get a cost more like:
1 * 1 * 2000gp * 2 (bags have no space limitation) = 4,000gp
Which is still probably too expensive to be practical (why not just get another bag of holding to handle the extra coins?)
For a zeroeth level spell:
.5 * 1 * 2000gp * 2 = 2,000gp
The other way to approach it is fiat: In order to be relevant, the item has to have a cost below a type I bag of holding. Therefore the price has to be somewhere in the 1,000gp to 2,000gp range. So it is.
The problem here is that the formula is balanced on existing, challenge-relevant spell effects. It works much better for an amulet of protection from evil than it does for a minor utility spell. This is why, for example, the Feather Tokens don't have the extremely high costs that come from creating something for nothing.
Why is the bag of holding so inexpensive compared to the formula?
Because the bag of holding doesn't really make characters more powerful, but it does make them more convenient to play.
A character with a bag of holding won't defeat many more challenges than one without. If it was more expensive, players would have to spend a lot more time on mundane tasks, like getting the loot out of the dungeon (which can be interesting, but not if you have to do it every week).