Since Doctor Who returned the show's episodes have been made available in "Series". There are Series 1 thru 4. I have not found them broken up and offered as individual episodes.
Then it was made known that this was the end of David Tennant's protrayal and there would be only 4 episodes. I think it took about a year before anything ended up here from BBC. I know they had it shown well before we got any news of it.
SO...when Amazon offered 2 of the 4 episodes last summer to be pre-ordered...I did...since they were being released individually. NOW...they are offering the other 2 episodes to be pre-ordered...OR...get all 4 in a set and pay less than the 4 individual episodes. I think this is unfair since they "broke the mold" when they offered the individual shows instead of waiting or letting us know there would be "Doctor Who: The Complete Specials" offered.
So I have the first 2, and if I get the other 2 at the current price, I will have paid $50.96 instead of $39.99 for the set. (It started out costing a bit more for the individual episodes, the Pre-Order price has come down a bit...BUT not enough to be fair)
Actually, the first series (Ninth Doctor) was offered as individual discs, although I don't think they've done that since (except in the UK).
I think that this is just the price consumers pay for being early adopters. If it's important for you to have the specials as they become available, then you should be prepared to pay for the privilege. What irks me is that the box set disc of "The Next Doctor" will apparently have special features not found on the individually sold disc.
We didn't know they would be offered as a set. I did the same thing as C. Kidder. Didn't know more were coming at that time or that they would package it all as a set. What are we missing by buying the individual DVDs except for the better (?) price? And Amazon had a great deal on all of the first 4 seasons by selling them for $32.49 instead of $64.95 in November 2009.