It seemed a nice round number.
4 seconds sound clip from the Cool Hand Luke movie soundboard.
You can hear this line at 00:55:10 in the Blu-ray version of the movie.
Quote context
[...]
- Now, when was the last time you ever saw my boy throw up?
- Now, shut your mouth and give your money to Koko.
- Come on, now, right over here.
- Luke, what did you have to say 50 for? Why couldn't you say 35 or 39?
- It seemed a nice round number.
- Luke, that's money you're talking about. What's the matter with you?
- Yeah, well, it'll be something to do.
- Ten.
[...]