Like most Magic: The Gathering cards, the value of Gold Myr depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Scars of Mirrodin)
The fact that the second card is so loosely restricted, and Force is so universal (and technically hard-castable) makes it playable in any fair blue deck, and so gives a large portion of the format the necessary panic button to discourage super-aggressive combo
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This can +2 to draw a card and scry two which is nice card flow, and then -3 to exile the top card of your library and then make a number of 1/1 flyers equal to the mana value, which is also very strong in Type Four with its elevated casting costs and low land count
It's quite mathy, but he goes into a great deal of detail and explains his rationale, while not ignoring the realities of the game (there are one-mana Cycling cards, cards that fetch lands, cards that tap to add mana which aren't lands, and more.)