Comparing the Coinbase market cap and the total market value of Bitcoin is not comparing apples to apples. Just in the last 24 hours $74B worth of Bitcoin has traded hands. The entire purpose of Coinbase is to facilitate that exchange and take a cut for doing so. If the trade rate doesn't change the entire "market cap" dollar value of bitcoin will trade hands every two weeks. Being responsible for facilitating a sizeable percent of those transactions is pretty awesome and very valuable.

With that said, I think they are overvalued at $100B. Not because of the total market cap size of crypto, but because their revenue doesn't justify a valuation that high.