I would say that while it's a pretty good find, there are two strikes against it:

1. Condition, condition, condition.  The most valuable signed items are those that are in prime condition.  With the cover being seperated as you described, that will lower the value rather significantly, especially if it's square bound vs. saddle stitched.

2. Personalization: If its personalized it's harder to sell, and consequently carries a lower premium.  Ideally, the only thing that should appear in there is the signature and nothing else.

As to value...well given that Runequest 2nd Edition books tend to go for rather hefty prices even in paperback (approximately $25 - $40 US), your tome, in premium condition, could easly net a 50% increase over that.  However, the two factors above conspire to hurt the value.  I would say that the book would carry a small premium, perhaps 10% at the most above the average RQII price.

Oh, and somewhere around here I have the first 5 traveller manuals.

Traveller wrote:I have a true first printing of the DDG.  Not nearly all that scarce.  I thought it was worth the $30 I spent, but it's just not that tough to find.

There's two or three listed almost every week on eBay...$30 is about right for one in VG.  I got mine for $7, but it's pretty battered.  (I just wanted to finally read those 2 mythos, so I didn't care).