Buying nymps-no.
Buying rares-eh...
The idea is great (Your art is also great Ajit), but the thing is, EtG has such a small card base, there are only 3 rarities of cards, can buy from bazaar, can't buy from bazaar, ultra rare. While this might be a good idea for newbies (oh boy! Random cards!) it doesn't really serve much since nearly all cards can just be bought from the bazaar. Also, that expensive? No newbie would pay 600, and no experienced player would either.
... Ya, I know nymphs probably were a no go, but they make a good example for ultrarares, along with shards.
I see the point on the 600
cost being too high... I've been playing for a while, but I don't have the raw data compiled to know a really good scale here, this was just a guess / suggestion... A good place to start calibrating would be to figure out:
1) How much electrum the average "common" card costs
-> Using entropy as an example: Bazar costs = {59, 29, 106, 56, 25, 24, 56, 100, 62, 59, 57, 25} (no pillar / pendulums), Average = 54.833
2) How long does it take to win a rare form the various AI / Arena Levels
3) How much electrum could be earned from "grinding" for the same amount of time as it takes to win / grind a rare at each level.
(no idea on 2 and 3 ... I'll need some help from others on that one)
Boosters give an additional place to start adding new cards. It gives a way to add in rares that fall somewhere between the level of the standard "can't buy in bazaar" level, and the ultra rare level. This can be further adjusted depending on whether
1) They are available from arena grinding
2) They are available from HB / FG grinding
3) They are available from PvP
4) They are available from Oracle
Adding in new, cards as booster incentives enhances the "Trading Card Game" feel, and depending on the rarity of the added cards, can also help to promote "Uniqueness of accounts"
In fact... you might expect more nymph-like cards in the future, cards that will do something that is already in the game, but in a different way. My goal is to have people actually creating their very own deck instead of everyone playing with the very same deck. That means that there is a price to pay: the chance that you will never get to have a specific card -ever- because it simply is too rare.
Boosters would give another way to distribute new rare cards.