How expensive? Remember that the Roblox Corporation does about a billion dollars in revenue per year and takes about 50% of all revenue developers generate on their platform.
Right, outages get more expensive the larger you grow. What else needs to be thought of is not just the loss of revenue for the time your service is down but also it's affect on user trust and usability. Customers will gladly leave you for a more reliable competitor once they get fed up.