The simple reason is that the development cost of D3 was far, far higher than that of TL2. If you have a look at the credits of both games, you will see the difference in the number of people involved. Also I know TL2 uses the Ogre engine (at least TL1 did), which would save the TL2.
But the thing is, you are right. Look at it from the consumers point of view - only immediate and obvious difference is the cutscenes, which I can watch on YouTube in HD. Other than that, similar number of characters, graphics look similar except for the style. So if I was looking to buy one, on what basis is D3 worth 3 times the money. Only 2 reasons - the name, and the fact that because of that name, more people will be playing the multiplayer. Number 2 matters not one white for me, I would rather LAN with friends. Or online multiplayer with friends, whatever.
Thats what I think the problem with the gaming industry is - the business model they use is not very clever. They invest so much into a game that they have to do all sorts of crazy things to break even.




