When Tarmogoyf reached the $50 mark, players heralded it as the end of days. Never before had Standard seen a $200 playset - even when a single deck dominated the format, the key cards never reached past $25. A card that was vital to every aggressive deck shattered that mark and set a dangerous precedent.
Tarmogoyf's $50 price tag came from two major contributing factors - its cross-format viability and its raw power. What was originally assumed to be a 2/3 for 1G ended up being a 5/6 more often than not, and that kind of efficiency appealed to more than just Standard players. The third factor, the fact that was printed in a "third set", contributes more to its price than most people realize.
The phenomenon of the 'Third Set' isn't altogether foreign to Magic players, but its effect is far more powerful than most people realize. Consider this - When Shards of Alara came out, people drafted three packs of Shards. Then, even when the luster of the new set wore off and people ceased cracking boxes for sport, people still drafted triple Shards. That entails a steady supply of Shards cards to the market.
If you've noticed that despite its constant use in Tier 1 decks, Ranger of Eos has yet to reach "elite" rare status, now you know why. Shards was out for long enough to supply the market with more Rangers than it needed. Now, had Ruel's invitaional card been printed in Alara Reborn, it might not experience the level of resistance that is does right now. Why?
The third set phenomenon is to blame. See, when you have three months of drafting a set alone, then another three months of drafting it with one pack of the first small set, there's a whole lot of cardboard going around. Finally, when the third set comes out, everyone has everything from the first set they need. They're well on their way to finishing their playsets of their second set cards, and the third set is only drafted for a few months before the next big set is released. Beginning with M10, the Core set contains new cards and has become a popular draft format, which has interesting implications for Alara Reborn. Released in the late Spring, ACR draft was supplanted mid-summer by M10 draft. Third set drafting time was cut in half, since M10 was drafted until Zendikar's release.
Baneslayer Angel, Standard's new record-holder, occupies a very similar space to Tarmogoyf. Her raw power is unquestionable, as shown by Bram Snepvangers in the Worlds quarterfinals. She's playable in Extended as well as Standard, and so help us all if a Legacy deck ever sleeves up Baneslayers alongside Force of Will. There are two new factors to consider since Tarmogoyf was Standard-relevant. The first was introduced with Shards of Alara - Mythic rarity. Now, they said they'd never make a tournament staple a mythic rare, but it's clear they either bent the definition of "tournament staple" a little, or just mis-evaluated the card. Be that as it may, Baneslayer is here to stay. It would be a crime not to reprint her in M11, but we might see a touch of mercy from WotC and have her be reprinted as a non-mythic rare. I wouldn't count on it, but it might not be the worst idea.
The second new factor is the inclusion of new cards in the core set. The core set now functions like a second "third set" in the yearly cycle, being drafted from mid-July until the end of September. As previously mentioned, that's only half the time a normal third set is drafted. Boxes will be opened with the same frequency, in theory, but losing a huge amount of supply due to a shortened Limited lifespan will have very heavy effects on a card's price.
Baneslayer Angel is the perfect storm. She's got raw, format-defining power in spades. She's got cross-format appeal. She's a member of the anointed "Angel" tribe, which can usually increase a card's appeal and value by a significant amount. She's a Mythic rare, and she's a Mythic rare from a set that was drafted for 2 months in mid-2009. Most decks would be better if they ran Baneslayer. Many dealers are out of stock. The main issue is that there simply are not enough Baneslayer Angels to fill out decks. That the M10 shortage kept some players from buying as much product as they originally would have. In the simplest terms, Baneslayer Angel's demand is as high as any card's can be. The supply? Literally as scarce as a new card can be these days.
All that being said, if you've been asking yourself, "Why is Baneslayer so expensive?", there's your answer. $55 may not even be the worst of it. Baneslayer Angel is similar to Google stock. It looked expensive at its IPO, and everywhere else along the way, but looking back, it was cheap all along. It's hard to evaluate this card because we've not quite seen this combination of factors before. The closest example is Exalted Angel, which was released in a large set as a rare and had nowhere near the power level that Baneslayer does. It is my personal hope that Baneslayer's price stops increasing. It's bad for the game when the consensus best creature in the format can be priced in large fractions of a booster box. In most cases, you can wait out a card's inflation from hype but in this case, trading for Baneslayer at full price might be necessary if you plan to fill out a playset before States.