If that IS solid gold, and it IS the size of a standard plastic card, I'm going to assume the weight of that card is possibly around 400 grams (gold is extremely dense). That means it's worth around $16,000. Might be even 500 grams at most, so tops that card is worth is like $20,000. That's a lot of money for Microsoft to give to a gamer for a gamerscore. I highly doubt they'll spend nearly a million though...
In order for it to be worth $1 million dollars, you'll require 24,863 grams of gold, or 54.8 pounds of gold. A standard bar of gold weighs about 27.4 pounds. You need 2 bars of gold for $1 million.