1. Next time you plan to read and prep a module (or just part of one) you're going to use in a game, look at the clock when you start.
If it's something huge and long, just count the time you spent prepping the part you're planning to use and reading any other stuff in the module that was necessary to run that part.
2. Then look at the clock again when you finish.
3. How long did it take? Write that number down.
4. Then, after running that module (or that bit of it) count how many hours worth of gaming you got out of it.
5. Then, the next time you want to run a game, think of a picture or a short passage from a book you really like, even just a phrase, and sit down and write an adventure from scratch spending exactly the same amount of time you spent on steps 1 and 2.
6. Figure out how much the module or the fraction of the module you prepped cost. (Maybe it was free, if so, skip this step). Use that money to buy something that would make the game you are about to run more fun like miniatures or funny dice or beer or a punching nun puppet to hit your players with when the evil cleric attacks.
6. Run it.
7. Compare the results.
8. Write about the results in the comments to this post.