JMich: The -75% is only if you complete the collection with the game you wish to buy. If you only want to buy Theme Hospital, and it's not the only game missing, you have to pay $2.99 instead of $1.49 for it.
On the other hand, if there is someone who already has all the games, the price he pays for any of the promo is the -75%, or $1.49 for each game (who said being a loyal customer doesn't pay off?).

ISC: Well, I have all the games in the promo, but I cannot buy individual games at 75% off. The only way to acquire them with the promo discount is with the "Gift the entire pack" button... or at least I haven't found any other way.

JMich: As usual for this kind of promo.
Step 1: Add the game to your cart, it will display the "Buy One" price.
Step 2: Go to checkout, it will display the "Buy All" price.
Step 3: ???
Step 4: Profit