I'm sure that Pornhub got a look at the books and knows the amount of subscribers that BCC has. If they were looking to make back all the money in a year, they would need a mere 6,250 subscribers paying $20 a month. But they may not be looking to make it all back all at once. Keep in mind that the transaction is likely financed and its basically not PH's money in the first place. There is also the so called synergy and supposed increase in business for both companies. One last thing, website owners lie. While the value of the sale may be $1.5M, alot of that could be bullshit or stock or some other compensation.
Those numbers are so far away from reality. First of all, you are forgetting that affiliates take 50% of every join and most sites have 20-30% affiliate joins as part of total joins.
Second, most sites have a ton of pricing options such as 100 bucks for an entire year, special offers like half off your first month, etc. etc.
additionally you aren't factoring in production costs, hosting costs, promotion costs, admin costs, etc.
I also think the rule of thumb is that the sale price of your website is profit for 3 years. so 1.5 million would be 500,000 profit each year for the dude which is much more likely.