If I follow your logic then, if I paid for the most expensive item, I should get the most expensive item. Correct?
When I bought the unlock the price "clearly stated" was 2 million credits or 475 coins for the character version and 1050 coins for the account version. I bought the account version so by your own arguments I had every reason to expect I was unlocking "bay 5" for all of my accounts. Unfortunately this isn't what happens: you pay the most expensive price and get the least expensive item for each character. For some this might be "bay 5", others it is "bay 4" and for the rest it is "bay 3".
Moreover it is my impression that only the price in credits changes: the price in coin is the same at all levels. Currently in game I can only see the coins price for "bay 4" and "bay 5" which is 475 coins for the character version and 1050 coins for the account version. However from what I can find on the net that is the prices at all levels. Consequently if you pay with coins your are always paying the "maximum" amount therefore you should be rewarded with the "most expensive" item.
Put another way my main and first alt paid 440,000 credits and now have 5 bays. My second alt has to pay a total 2,040,000 to get to the same position (he'd already unlocked an additional bay) while all my other alts and every new characters will have to pay 3,000,000 to get the the same position. I don't believe it was BioWare's intention to make our later characters pay more to get the exact same things. Therefore I don't think it is working as intended.
But conversely, if you had been on an alt when you bought that account wide cargo bay, you would have gotten a huge deal, the 2000000 credit bay for free. The price per bay is already set, no matter how you pay for it. The second bay is 40k, third 400k, etc or cartel coins.
I personally think the cartel coins is a better deal, even if it does only save me 40k per toon, because any and all future toons i create will already have that tab unlocked, and once i buy all the bays with cartel coins, i should never have to buy another cargo bay tab again. The same theory applies to inventory space.