I regret not getting my hands on the Sorority House Massacre one sheet when I saw it available! You're lucky to have that one! The film didn't really catch my interest as much as the Slumber Party films so I just let it go...but it is a really good poster!
It seems to be a less common find along with SHM2, which I'm assuming is much rarer!
As for the prices of Slumber Party's one sheets...I think they're just ridiculously overpriced. No one is buying them at those prices and they're not even rare...you can usually find listings for the first film and they clearly aren't selling since it's always from the same sellers. I'll admit that I probably overpaid for mine...but it was a bidding war and I really wanted it!
The only one that seems to be really rare is SPM2's theatrical one sheet. I haven't seen a single one of those, while the small video poster has popped up a few times the last few months.
Overall the first film's poster is much more common than the other two films...but SPM3 seems to be much less desired and thus priced lower. I got mine at a bargain and the current one I see for sale is priced pretty decently.
Hopefully we'll both get what we want at reasonable prices!
I looked them up on eMovieposter.com. S-lumber Party 1 usually seems to go for a good price ($100+), but not (you are right) what those on eBay are asking.
SLUMBER PARTY 2 and SORORITY HOUSE 2 have apparently only had 1 copy each of the video posters on eMovie. That's it.
SLUMBER PARTY 3 is fairly easy to acquire.
There is a SORORITY HOUSE MASSACRE 3. It's rather elusive as it is also called HARD TO DIE and TOWERE OF TERROR. I've never seen any of the SORORITY HOUSE movies and the third (poster and movie) seem incredibly difficult to track down.
I liked SLUMBER PARTY 1 okay. At first I thought it was ridiculously dumb...and then, while watching, I learned it was originally supposed to be a parody. You can SEE that if you are watching for it and, to me, that made it more appealing. SLUMBER PARTY 2 was the odd one---and the one i think I liked the best. But I don't quite understand what it is. Ha! The killer---a break dancin' rockabilly singer with killer drill in his very stylized guitar? WTF? Hilarious and bizarre. And...what? Is it all a dream, hallucination or what?
SLUMBER PARTY 3 seemed to be a just-in-name only sequel that was unrelated to the first two and only had the slumber party theme and drill killings to make it a sequel. Okay--but not really in the series, if you know what I mean.