The mono albums usually go for more money than the stereo versions. They were mixed/mastered differently than the stereo versions and are more rare.
Here's a web page explaining all the mono stuff: http://www.deadwaxcafe.com/vzone/beatles.asp
Of course any "special" Beatles records, like fan club issues are more rare. I saw an unopened Help! album on ebay the other day and the dude wanted $1300.00! Shameless blog plug: http://vinylcast.net/?p=540
I have one of those record price estimator books, and there's like 5 pages on Beatles albums. They get into the different colors of the labels, and serial numbers, etc... but the general rule is mono or some sort of rare collectible like a fan club issue will fetch more dollars.