I vaguely recall spending about $20-30 for the two Realm of Chaos books when they came out approximately twenty years ago. (Of course, they got thrown out by my parents during the great RPG scare, but that's another story...) The last two Realm of Chaos: Slaves to Darkness auctions that I remember seeing went for about that much, with Realm of Chaos: Lost and the Damned fetching more by virtue of being rarer.
As a random comparison, I paid about $5 + shipping for my used copy of Codex: Chaos last year.