I hate to say it still, but it just depends on those different factors as RayB stated.
For example I got a steal a month ago on eBay for my 4 slot... 630$ with 18 games and one of them is rare. Just because the guy listed things poorly and it was originally listed local pickup only. He turned away a lot of bidders initially. I'm sure the auction would have gone over $1k if he described it and the games better. The Neo Geo collector crowd will pay some insane prices for items in mint shape.
Another thing to consider with your situation, you will want a 4 slot model. It is an ideal cab for what you want to do. If you don't get one you will probably get tired of changing out games... especially with your idea of a home arcade for people, you will want the selection. Nothing ruins the illusion of creating an arcade experience like having to swap out games in front of people, it just starts feeling like an overgrown console. At any given time in my cab I usually keep a shooter, a sports, a puzzle and a fighter game... works out great.
In general the dedicated Neo cabs are more desirable and their prices will reflect it. 6-4 slots usually go for the highest prices. Nice 2 slots can be almost the same price because they are just as popular for the smaller form factor. Most of the 1 slots will be conversions, the one and two slot models also could have only 19" monitors in them...
I think someone here recently listed a 4 slot in decent shape for around $500 in the B/S/T forum. It might still be available? I've been staring at prices for awhile and it can vary greatly depending on everything as previously stated. At least with the 4 slot model all of them should be 25" monitors and the hardware is desireable so that keeps the prices consistant.
Anyways I think around $500 would be a reasonable starting price for a 4-slot in good shape, with the 25" monitor working well. You might get lucky at an auction though and snag something for cheap or have some rare or high demand games included with the listing, all which all effect prices too.
Just for you to be prepared that model is very very heavy and awkward... I'm never again going to risk moving mine until I strip out everything. I don't care if it takes me two weeks to do it and I move it piece by piece. I about killed myself. You will need a refrigerator dolly also, those small ones wont cut it with that beast.
I'll probably edit this later... I have to finish up some work before I leave for the day and my thoughts are a mess. Sorry if it's confusing.
-baker