It’s due quarterly. If your profits are under a certain amount you don’t need to pay anything. I’m not sure what the exact amount is, I think it’s $2,500 a quarter.
Think of it way. You get to use an engine, that in the past required a minimum of about $500,000 to get into for use on 1 game for a 2 year development cycle including the maintenance fee, for almost free. $20 a month is practically free, look at the subscription price for Adobe Creative Cloud or Autodesk.