Worst case scenario, you spend 4 hours between communicating, negotiating, gathering files, designing the flyer, getting feedback, making changes. etc. Best case, 1-2 hours. $50 isn't even close to enough, $100 is better, but not enough to sustain a business. If they won't pay $100, they don't want the flyer that bad.
Bigger picture, hourly work is for employees, and/or deep pocketed clients willing to pay $100/hr. +. Most people will appreciate a flat fee, and you can get paid more per hour by working smarter, and having a clear contract. If you charge by the project and know what you are doing, you can charge people $100/hour that would never have agreed to pay $100/hour.