Another top tip then! When you do find it bid 502 on it !
haha.
anyway whats the most you have ended up forking out on something?
I just got the hexen 2 add on pack portal of parvaious for £12 GBP
Really I always bid 503, that was just to throw you off. (hehehe, just kidding)
The point is that I realize what in my collection is important. I try to collect most any DOS and pre-WinXP First Person Shooter. Like DooM games... one of the first "add-ons" I ever bought for any game was D!Zone 150. It was AWESOME to me! As the others in the series came out, like D!Zone 75, D!Zone 2, 3, Collectors Edition, D!Match and so on, I bought most all of them. So now I have most all of them, in most box art variants even... only one left, and it might not exist, D!Zone 2 150 Boxed. Long story I posted in the Does it Exist thread. If I discovered another D!Zone I didn't know about, I would easily bid $500+ to have it for my collection.
It might work the same for Build Engine games I don't know about. I have at least some of every Build Engine game that I know about. If I found out I was missing another, I would also bid a great deal to have that in my collection as well.

I snipe to avoid bid wars, so I never seem to pay too much on eBay. The most I ever paid... on eBay it was probably Duke the Apocalypse 2. I don't think it was ever released in the US... so I bid a lot on it. Maybe $500, I really can't remember now, but I got it for about $150. I don't collect perfect games, so I seldom go very high with bidding unless it is a specific game on my top five wanted list. Even that isn't very high sometimes, but I do actually budget each individual game... some games do have "max" however... whatever I am willing to do at the time.
Number two on my list right now is Boxed Deer Hunter Screen Saver... it won't cost much at all when I find it, but I have not seen one in years! I am the only collector I know of that collects the Deer Hunter Series... they value is quite low, which means nothing to me. I collect games because I want them, not because of what they could be sold for. Deer Hunter game series has grown to about my third largest collection series actually.
I guess what I am trying to say, is that it isn't what something is worth, but its value to my collection that sets my top end bids.
That is a really good deal on Portal of Praevus it sounds like.

For a while there it sold in the $40's or more (USD), but that was about 4 or 5 years ago.

MrBlackCat