Not trying to be sarcastic, but I think the bottom line is "Depends on your accountant and tax attorney." I made much less than a friend and paid more tax, got a lower "stimulus" check (didn't stimulate me very much. LOL) etc. That's because she has more deductions.
I think $15M is a bit high. That's what you'll get paid, but you will probably owe about 5% more. Another poster said in a different thread it will be around 35%, but it might be lower if you find someone who can offer good financial planning. So my answer is, after all taxes are paid (unless you live in a state where the lottery prizes are taxed)* a cash prize of $20,500,000 should leave you with about $14 million.
Does it really matter? $10 million, $15 million, $20 million .... Who cares? Just show me the money!!
*You get taxed where you legally reside.