When do you really not need a CDN for static stuff?

Everyone talks about CDNs for fast static content, and yeah, that's smart for global apps. But what if your users are all in one small area, or it's just an internal tool? Does a CDN just make things more complicated and expensive then? I've seen smaller projects, ones without a ton of static stuff, where getting a CDN going felt like way too much hassle for what it gave you. Is there a point where the speed boost is so tiny it's just better to use one good server nearby, or even just let the browser handle caching? How do you figure out when a CDN isnt worth it?