The canal was blocked for 6 days and revenue is $15 million per day, so that's $90 million. But many of the ships that were held up eventually transited anyway, so $90 million is an absolute upper bound. Then there is the cost of the operation to free the ship, which I don't know. But Egypt is asking for $1 billion, so the rescue operation would have had to cost at minimum $900 million for the price to be justified. And I know it didn't cost $900 million, because if split evenly over 6 days that would be $150 million per day, which would be > 50% of Egypt's entire government spending over the time period. So I can say with confidence that the rescue was at least an order of magnitude less than that, so another $90 million (and that's still a massive over estimate).

So absolute upper bound for losses is $180 million. If I were making odds I would guess that only 1/3 of the ships diverted around Africa, so $30 million, and that the rescue cost another $15 million, so over/under $45 million.

And that's without even bringing up that there was a canal pilot on the ship at the time who may share some portion of the responsibility for the accident. So, yeah, it's extortion.