EDIT: And it's so expensive because a few hundred cartel coins = a few cartel packs = 250,000 credits each = 1 million for 4, so basically they made the costs equivalent. Also, they want you to pay RL money on cartel coins, instead of being F2P and taking up server bandwidth for free 17 hours each day so you can grind 1 million credits by farming heroics or flashpoints or whatnot.
Unfortunately, there's really no direct correspondence bewteen what it costs to buy legacy stuff with cartel coins vs credits. The ratio of coins:credits is not remotely consistent (IOW things aren't priced at 100 coins = 100000 credits or whatever.) Many items have high coin prices but relatively low credit prices, and vice versa.
And there's even less correleation between what it costs to buy stuff from the cartel market (coins) and what it sells for on the GTN (credits), especially packs and the items from them.