I think we already have a high and low market price to each card next to it. On the official Steemmonster market you can see the high and lows offered by the sellers.
But one more important thing is, when you say the sphinx is selling for 200$, that just means somebody has a sell order for 200$, there is no evidence that this sell order will get filled. Maybe most buyers think that this is over priced and they won't buy until the price drops. From market perspective the most important information is what was the price the card was traded last? We need an order history to each card where we can see at which price actually a trade happened.

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