I have this question, too. I was advised by someone at Mac App store to purchase OS Lion 10.7 for $ 19.99 which I did. Only to find out after the purchase and download that I only had 1 GB and needed at least 2 to use Lion. Now Apple refuses to credit my account for the $19.99 charge sending me some sort of coupon, but cannot be used to purchase the ram needed for my IMac. nor when I do increase the ram, I would not be able to re-purchase Lion.
I was wondering about Maverick or Yosemite on the older IMac. They are free. Don't understand why the older versions are not free, also. I have Yosemite on my MacBook Pro and like it very much.