I'm a big fan of modded production knives. The below Boker Mini Kwaiken and ZT 0450 had regrinds by Josh of Razor Edge Knives and aged micarta scales made by the infamous Steve Ketchen:
To me, even when the costs of the mods exceed the price of the knife itself, it's justifiable when you really, really like that particular knife and there simply isn't something else in "stock" form that could replace it for you. This is especially true with mods that dramatically increase the actual usefulness of the knife as a tool, like Josh's regrinds.
A few more while we're at it:
Small Sebenza with aged paper micarta scale by Ketchen:
Boker Zero with regrind and acid wash by REK:
Spyderco Roadie with regrind by Josh (which turned it into the closest thing I have to a scalpel):
Sometimes the modding can get a little freaky. This particular one was a Chinese clone of a RAD Knives Field Cleaver that was completely reshaped (blade chopped down and handle shortened and reshaped), with a micarta scale, then re-tapped for a deep carry clip: