so, the gold cards or whatever are worth 10k G each. G and Yen have an exchange ratio of 1:1, meaning 1 G = 1 Yen.
Then he purchases Furnature for 397k Yen. That's 10 times the amount of money that he got from the 10k G thingies.
And then they also go out to buy groceries and eat lunch... Or were thes cards actually worth 100k G each and he can easily get groceries and lunch for 2 people for only 3k G?