I went with $2,500.
But, as others have mentioned, it can be a situation-dependent kind of thing.
I'm in to one of my ARs for about $1,800. And although it has a lot of high end parts and shoots as you would expect for an 'expensive' AR, I'd never be able to sell it for more than maybe $800. It was 'expensive' because I wanted a very specific combination of parts. But its market value is much lower than my investment.
My father's .450 Nitro double rifle? Expensive.
A Ruger 77 Mk II? Not expensive. Just not cheap enough for many people to be able to justify, when looking at racks full of modern budget offerings.
Some things are just comparatively expensive, especially when 'custom' comes into play.
I have at least $1,400 into a Marlin 444 that I built on a 336 action. For a run-of-the-mill 336 or standard 444, that's expensive. (And would have been easily $4k+ if I had a 'smith build it.)
Sticking with Marlins, I have a .30-30 336 that is nearing completion. It started as a sub-$200 basket case that was left to rot. Every part of that rifle has been modified, replaced, tuned, polished, or built from scratch -- or will be before it is finished. The sights are one-off parts. The fore-end tip cap was a custom design. The magazine tube is a one-off. The mag tube plug is completely custom. The lever is a one-off. Magazine and barrel length are non-standard, with a custom crown. Etc, etc. Even some of the screws had to be custom made.
Not even counting prototype/proof-of-concept parts or 3D printed test parts, I've already spent well over $2,500, and still have some expenses to go (such as possibly redesigning and remaking the as-yet-untouched custom butt plate). For a .30-30 Marlin 336, that's damned expensive.
...But, as Scorch put it, that's just 'entry level' for most customs. The only reason I can afford this project, amidst everything else I'm working on, is because I'm doing 95% of the work myself, and specifically designing the one-off parts that I can't make myself to be easily machined (reducing costs). If I sent the rifle to a custom shop for the whole job, I imagine it would easily hit $5,000 with refined factory or aftermarket parts in place of some of the one-off parts, and over $10,000 if all of the one-off and custom parts were included.
Certainly an expensive rifle, by most standards. Very expensive for a Marlin .30-30.