Here's simple check for you: I calculated average last price for all Elemental Rares - result is 104.05 g per rare. Just posting some real value for comparison.
(I was too lazy to calculate it more accurate and do that for other domains
)
Calculating that value for all cards will give you pretty accurate market price of Cards by rarity.
Also important thing is that average market price and average value of card is never same.
Your math looks good, but the point is that it is correct only if we have only 1 rare card in whole game, so you always getting it from pack and you don't need any specific one, don't get good rare that doesn't fit your collection and so on.
Another case when your math might be correct is when random generator is always gives you different cards, so you can buy 417 (139*3, 139 is all rares in Seed and Borderlands set) packs and get 3x of every rare card in game, but real result will be much worse.
I made few tests - got next amount of rares from 417 packs:
0x rares: 5%
1x rares: 15%
2x rares: 21%
For 900 packs:
0x rares: 0%
1x rares: 1%
2x rares: 3%