They paid that because they did their sums and were confident they'd get a return. They would've paid 300 billion if they were confident they could get a load more back. That's a little harder to gauge with BTC.
So, did the NYSE do their sums before investing in CoinBase? I don't see them getting a good return without the bitcoin exchange rate increasing a hell of a lot.
I assume they're looking to the future, but there's plenty of money to be made in the turnover of BTC as well.
You could have a very busy economy with a relatively puny market cap with the way it's running at present. The coins are effectively acting as tokens for USD when it comes to most merchants. They make their money facilitating that conversion.