[bradlane]

because your calculation assumes that out of 100 flips, heads always gets 51 out of 100, which is not how probability works - it's not linear (for lack of a better word).

It's possible to be up $10 in just 10 flips, just as it's possible to be even at 1,000,000 flips (though neither very likely, but possible!). Also, the question asked "with a 99% certainty", which your answer doesn't take into account.

If you had a coin that when flipped had these odds, and you wanted to try this out, your answer of 500 would be a good rough estimate, but not the answer.