Collector interest, PERIOD.
There are plenty of prototype and low production games that are worth less than $2000.
Many people have never even heard of at all.

An example not included.
King Kong (Data East, 1990) - 9 units (supposedly)
UberRare, Expensive, and coveted by owners.
Many will not even say they own it.
Why?
Artwork and theme.

Most of the data you require can be extracted from the IPDB.
It is already there.
If you want pricing information, it is all over the map.
If a collector wants it, they will pay the price for a rare game.
Many times private sales remain just that, private, there is no data.
Last time I say a KK for sale it went private sale for over $20k, which was insane at the time.

There really is no "scientific method" to understanding pinball collecting and its relation to rarity, cost, or interest.
Its like any other hobby.

"One person's junk is another man's treasure".

Actually this game is for sale if you want to make Denis an offer. Has been for a long time. The art is hand painted and not sealed. The cabinet hardware is all gold plated. The code is unfinished and recycled.

There are other games with production runs of 1-2. Gameplan Loch Ness Monster for example. Phenomenal backglass less than 10 spares were made.

Although not directly related to this posting in terms of pricing, something that will most likely be overlooked is sample and test games.
I realize that these can be classified as pure prototypes, but many were made in numbers between 10-100 but generally never more than 1000.
Many collectors prefer these type of machines either from the standpoint of features not included in the production machines or rarity.

Although I could list many, one that comes to mind for me personally is Doctor Who (Bally, 1992).
In this regard, I am not speaking about the "Wobble Head" versions that most may be familiar over the recent years.
This was a "Version 2" sample and started production game.
In this case, I am referring to the "Version 1" prototype that used the Bally '89 cabinet style with the speakers on top.
This version included a lithograph hologram in the certain of the backbox. 10 were made, 2 exist left to my knowledge, after most were scrapped in favor on the standard cabinet style of today, when they were sent back to the factory for revision. Very rare, very collectible.

Just like John said, games like Charlies Angles (EM version) are sought after by others to no end.

Just like turds in public toilet, right?
They are very rare too.

Anybody want to buy my backglass?
(I am joking, no PM offers please for my glass)
10 spare BGs were made to my knowledge, would make a fun reproduction though. One came up on ebay quite a few years ago. I was told another actual machine is floating around the world but it's never been confirmed, just the one everybody knows about.

Another machine not mentioned, but should be.
Krull (Gottlieb, 1983.)
10 units, superb System 80a machine with magnifying lens window on the upper playfield.
The production number COMPLETELY confirmed.
Everybody it possible should get a chance to play this machine if possible once.
It is like a cross between BH, HH, and DD.
Licensing issues killed this one.
Another example of bad movie, but a great pinball machine.

For those who have forgotten how crazy things can get in the pinball world.
This was a legitimate auction.

ebay.com link: King Kong One of the rarest games in history

It plays very well, the software game code was unfinished however (unless John decides to do code...), that is why people say, "it plays like $#%@".
That is an unfair judgement for a prototype.

I only know of 4 of the owners, and they are not selling, although you would be surprised how many that ended up like this one in 2013, in storage (climated controlled, wrapped, and protected of course).
The game is, what it is, a straightforward playfield with good callouts, and simpler features.
Complexity is about the same as RoboCop.
Backglass artwork was always admirable, I do not remember how many extras they made but there were very few.

Data East could have had a real winner on their hands.
Reminds me of the original BBB days.

John,

I always wondered why you sold your original Fireball?
Always seemed to suit your collection.

Another pinball secret revealed after so many years! Just been around for a while...

One word, "parts".
Electronics especially, but things are finally improving.

People need to collect what they LIKE.
If it's RARE so be it.
A collector pays for what he/she THINKS it's worth.
Sometimes more, sometimes less.
I like KK, but I dont want to pay $20k, which is MY choice, regardless if I can afford it.
It a hobby and a toy.

There is an old adage, "just because you can, does not mean you should".

BTW, always liked Joust and Krull.
Loch Ness is ok, but I really just like the backglass (it looks great on a wall!).

Steve,

Never say, never.
If you want it, it will eventually happen unless there is "one prototype" of a title.

I may take 10-15 years, but it will happen.
I have found every single machine title I have ever wanted over time.
Sometimes they even came in pairs!

Some titles, people would just not necessarily want to keep, if they are hand painted, and your really cannot play them with clearcoating first.
Loch Ness is a tricky one for sure, and I still never received confirmation there was two FULLY WORKING machines.

Like you I once owned every single Gottlieb System 80 machine (except Krull), and then slowly passed them onto other Gottlieb collectors.
Devil's Dare was one of my favorites.

"The truth is out there".
It depends on how much of your wallet you are ready to empty.
Ten years ago, you could probably get away with $6-8K.
Now, it will be a little tougher, add maybe $4K.
Not sure if the people I know want to sell, rock solid, and hardcore.
Most have been "revamped" with modern replacement boards to get rid of those nasty Gottlieb edge connector issues and some rare ICs, but kept the original boards in boxes.

I though it was a Dayduff "WTB King Kong" thread?

What is that mounted wire in the center lower cabinet?
Is that part of a grounding strap?