Here's what came to my mind immediately when I read your question.
To me, the obvious answer is, it depends on the person interested in the cue. As much as I love this game, I can't see spending more that a few hundred bucks on a cue no matter who made it.
As beautiful as some of them are, spending money in that area has never even been a conscious thought to me in the 50+ yrs I've been playing.
In fact, it's just the opposite. If I ever spent a lot of money on a cue and I didn't like how it felt, I'd be sick. I'd rather play with some cheapo cue that feels good, than risk a crap shoot on buying a custom without playing with it first.
That doesn't mean I don't appreciate the workmanship and the fact that some folks like to collect cues btw, to each their own.
Since the idea of a letter would be a collectible thing, then again, it's in the eye of the collector. No different than someone all excited over a painting. To one person a signed painting is a big deal, to the next, it's of no interest. Or like a guy who hand builds a car in his garage, the zillion man hours, blood sweat and tears, and expense of building it put collectible value to it. But, it doesn't mean it's a better car than grandpa's Buick. :shrug: Just more collectible to those interested.
Find the right collector and it has to be worth something I would think.