I would rate both versions of
Tome of Horrors in the $20-30 range, with the 3.5 version being slightly more valuable.
(But...am I right?
Did the 3.5 version actually exist in hard copy form?)
I have kicked myself several times that I let a copy at Half Price Books go when I could have paid only $14.
As far as I can tell, Necromancer Games items in general are going to be more collectible than a number of other companies because they have some really choice old-school connections, including
City State of the Invincible Overlord, the
JG Wilderlands property,
The City of Brass, and the First Edition goodness of the
Tome of Horrors series.