I just figured up my total costs on the Artic Mini Mame (now that it was finished), and they came out slightly negative (my leftover parts are worth more than what I spent total).
$13.00 Cabinet
$75.00 Monitor
$10.00 Computer (multiple trades, was essentially free, the money was the shipping I paid))
$ 3.00 Interface (joystick port)
$16.00 Ms. Pac/Galaga replacement stick
$ 1.00 Buttons (already had, I just gave them a value though).
$ 1.80 Control panel overlay (rubber stair protector from hardware store).
$ 0.50 Speakers (garage sale)
$ 5.00 Video card (voodoo3, already had, just gave it a value)
$ 0.25 Keyboard (garage sale).
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$125.55 Grand total
Leftovers (and their guestimated value).
$100.00 Unburned, almost unused (super clean) 19" arcade monitor.
$ 10.00 Original control panel overlay in mint condition
$ 3.00 Original 4-way joystick
$ 5.00 Audio amp
$ 7.00 coin shroud and speaker
$ 3.55 wiring harness that no one will ever want.
$ 5.00 Transformer
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$133.55 total of leftovers
Total cost ended up being negative $8.00
That list is a bit slanted IMO.

First of all, you consider things as "free" if you traded for them. They aren't really free, as you traded something for them, and the things you traded must have come from somewhere. Be it an old computer part or whatever, you bought that computer part once upon a time. Even if you traded a part that you traded for another part that you traded for another part, the stuff came from somewhere. Unless of course you traded a part that someone just outright gave to you. That Voodoo3 card might be worth more than $5 (I don't really know).
Maybe this is a list of "total cash outlay for the project" which could include a $500 PC if you already had it, or many expensive items. If your purpose is to show how much you actually spent on the project, then this is acceptable. (this is what many of us do since we have lots of "extra" parts laying around anyway that are, for the most part, worthless).
You don't consider any of your time, effort, travel, etc. in these costs. I could say, "I got this pacman cocktail for free." But in reality, I did 10 hours of work for him that I usually charge $100/hr for, and he gave me the game in exchange for the work. Did I really get the game for free or did I basically pay $1000 for it?? Saying I got the game for free would be misleading or optimistic, saying I basically paid $1000 for the game would be more accurate. I do this kind of stuff often and I always treat it just like I actually paid for the stuff.
Also, your list of leftover, valuable parts is really just a list of other things that are practically worthless, too, with exception of the monitor. Is that overlay really worth $10? How many people are looking for a King and Balloon overlay? If that Audio amp were a PC speaker amp and you used it in your Mame, you would have given it a value of $0.00 or next to nothing. Even the monitor is estimated high, IMO. Sure, a new 19" is more than $100, but if you were trading it for other arcade parts, or throwing it into a game with a dead monitor or whatever, it would be more like $50.
I spend a small fortune on my first Mame box, but I had to have everything just the way I wanted it, and everything brand new. I will probably build more of them in the future and they will definitely cost a fraction of my first one. If I'm just going to give them away as gifts or whatever, there is no sense in going "all out", but I wanted to do that for my personal machine.
Not to shoot down your list, I'm just saying I think it is "optimistic." No doubt you built your mame for very little money and found great ways to save money. And I think that was really your point anyway - that a Mame machine can be built for very little money if a person really wants to. And I agree.
Wade