That one's not worth a lot I 'm affraid... if you'd have the Halloween II 12" or the withdrawn version of Halloween with Satan is Boring (supermix) on the b-side then it would be a diferent story.
There's a database of auction results for vinyl records at
www.popsike.com.
Not something you can totally rely on because question and demand fluctuate and the results can skyrocket just because of 2 people wanting the same record.