They're up on Google Play too. I got the Slings and Arrows books.
I've noticed a lot of series actually do seem to do this, the first one or two books will be really cheap, like $1 or $2, but they get progressively more expensive until you get to the latest book which is usually the full paperback or hardcover price.The cynic in me want's to say that this actually is a good business tactic. "Accidentally" give the people part one of an old trilogy at a reduced price, and if they're hooked they'll buy the other two for the original price.