Is it cheaper to get a guitar refretted or just replace the neck?

It would depend on what was involved in replacing the neck and the quality of the replacement. If you had a Strat or Tele type guitar you could probably find a relacement neck for maybe £/$30 - a lot less than a re-fret (although that might just be the start of your problems).

On the other hand, replacing the neck on a guitar with a set-neck would be a lot more expensive than a re-fret.

It's much cheaper to get a guitar refretted. A neck replacement is a much bigger deal and is not recommended unless there's an actual problem with the neck.

Edited to add: Tony knows a lot more about this than I do. I was thinking of an acoustic neck while he's right that many (most?) electric necks just pop off easy-peasy. But you still need to consider the cost of a replacement neck versus the cost of refretting.