There are two strategies here. One is to try out as many keyboards as possible that are weird or interesting for the experience. That's me and Dorkvader.
Most people have more or less decided what they like, and are willing to plunk down a lot of money and time to outfit one board to perfection
I'm actually starting to move from category 1 to category 2. I'm deciding what I want, and thinking about selling some of more "normal" keyboards to pay for it.
as to the most I've put into a KB: umm I dunno. Probably my skeldon (Proudly made in the USA! )
There's the price of the kit (lots)
the phantom PCB (I dunno were they $40 or so during the original run I think? my other phantom PCB is from MK)
MX white switches (from 7bit's GB back before we even knew what they were)
Titanium spacebar (about $110 IIRC)
Keycaps were from a NIB G80-8113 that I got on here for $100. Not really sure what portion of that counts though. What's a NIB set of cherry doubleshots with center stemmed capslock worth?
A lot of my keyboards you can't really estimate the worth, as I received parts in trade for labor, or as gifts. Many keyboards I acquired for significantly less than they're probably worth. Some the value has risen with time, some the value has dropped.
For me, it's all about the hacking.