If it were mainly about lumen output I'd agree, but it's not. First, if you are talking about a single cell flashlight, the runtime is not going to compare to a 4 cell bike light. Most of my brighter 18650s only have 50-80 mins of runtime on quality cells. Since the OP goes to work at 8pm, he could end up needing to use it both ways and one cell may not be enough.

By time you pop for a decent light that wont shut off on bumps, wont overheat and is reliable; add in extra cells so that you can always have at least one cell fully charged for use the next day (perhaps more if you want to make sure you have the To and Fro both covered and in case you forget to plug in the charger when you get home); add a charger and mount and the cost can exceed than that of a MS.