User spends $100 USD to buy 10000 robux. User buys item for 10000 robux in a game. Developer of game gets 7000 robux (dip #1). Developer of game withdraws to USD. 100000 robux = $350 (dip #2) so developer gets $24.5 USD.
Overall 24.5/100 = 24.5%.
In reality the numbers don't always come exactly to this, because robux can cost different amounts to buy depending on the amount, and there are PayPal/wire transfer fees when withdrawing.