AMD wants to make money and whetever price they put on Vega 64, gamers will look elsewhere. Why? Because even if AMD decides to sell ALL Vega cards at their $399 or $499 points, Nvidia can come up with price reductions for their 1070 and 1080 models, making AMD bleeding money everytime they sell give away a Vega card. Even if Nvidia doesn't do a move, people will still keep buying GeForce cards because "they need less power and produce less heat". People will always find an excuse to go with Nvidia or Intel brands. If you don't believe me, just look how fast many people change their stance about Ryzen cpus, now that Intel is promising more cores.
Another thought. Let's say that AMD, or it's partners like Sapphire, sells the Vega 64 at $450 to the retailer and the retailer, because of mining craze, sells at $700. Who makes the more money? But if AMD, or it's partner, sells the majority of cards at $675 and only offers limited quantities at $499, then they can make some money. The retailers of course will come out crying because they wouldn't be able to enjoy huge(for an electronic device) profit margins.
AMD knows that Vega will sell to miners, to movie studios and professionals as Frontier, FirePro, SSG, Instinct, whatever pro card. And they probably already know that it is a lost cause to try to sell this card at significant quantities to gamers. So they try to sell Vega RX in a way that wouldn't cost them money and would help their partners that sell ONLY AMD cards make some money themselves. AMD puts it's financials and it's partners at top, leaving the retailers crying and gamers unsatisfied and angry. Well, I don't see something wrong here, at least from a business perspective. Gamers will come back when AMD will be ready to offer a true gaming card. For now they don't have the money to offer true gaming products. Both Ryzen and Vega are pro products, not so much for gamers.
PS I try to explain AMD's actions, NOT give excuses. They should have come out and say from the beginning that, because of the cost of the card to make and uncertainties with VRAM pricing, they can only offer limited amount of cards at discounts.