https://pbs.twimg.com/media/DYy--NiWkAA_E1U.jpg:large
They are kinda spammy looking, but then again that's a lot of paid emails, and you are getting paid to read that spam. You can count $15.50 in that screenshot alone. Multiplied across a userbase somewhere between 100k-1M (based on the quoted numbers of "hundreds of thousands of users") and assuming that most of those emails came within 1-2 months time, that's $90-180 per user x 100k-1M users. On the low end that is $9 million per year in total paid email volume. On the high end it is in the tens of millions per year.
There aren't too many public numbers beyond "hundreds of thousands of users" and "millions in revenue". We have to see what Coinbase does with the Earn.com product and where it is in a year before judging this acquisition.