I think a more relevant metric would be the velocity of the Cypto market rather than the total value of the number of coins out there. Since Coinbase is taking a % of every transaction, if you send .1 bitcoin once then they get .5% of that. But if you send .1 bitcoin 10 times then they get .5% of 10*.1 (not the real % obviously).

I've seen numbers around 250k to 350k transactions per day for Bitcoin and more than 1 million for Ethereum. So if Coinbase can capture a decent percent of that market then you're talking about tens of millions in revenue per day (if not more).

From a quick search it looks like Coinbase had around $1.8 billion in revenue in the first quarter. That could/should grow along with popularity of crypto in general.