My God am I late to this, but here goes. Firstly, yes it is the same engine but Bungie has stated repeatedly that it is a heavily tweaked Halo 3 engine, and after spending way too many hours in Reach (many more that in Halo 3 to be honest) I have to say that it really shows. Reach is undoubtedly better looking and sports bigger levels that H3.
As to why spend $60 on same old, same old. Well, many would argue it is not. In fact there are quite a few that claim that Reach is such a departure from the Halo concept that it is a Halo game in name only. Secondly, Reach serves a good story (much, much better than Cod:Blops IMO) as well as an amazing multiplayer platform, and considering the heavy work Bungie has put into the engine I would say that Halo Reach adds more bang for the buck than most sequels do regardless of genre.
I didn't really "get" Halo until I started playing Reach. I found the universe quite compelling and really enjoyed the multiplayer aspects of the game even though I suck badly at it. It really demands a lot of tactical analysis of the player since running and gunning will kill you pretty fast. The new armour abilities does add a lot to that strategic aspects as different abilities has different pros and cons. Finding a group of regular players makes Halo Reach into one of the best gaming experiences I've had in many years, flight-/subsims included. I've already spent around 80-90 hours in SP and MP and still play the game on a daily basis. I can see myself playing Reach for years to come, and if that isn't value for money, I don't know what is.
So at the end of the day I believe that if you take a closer look at Reach you will find that it really isn't so much more of the same as you might think. And on its own merits it is a rich and rewarding multiplayer/social experience that knows how to entertain and captivate its players, and that goes a long way in my book. To each his own, as always, but I thought I should I should chime in and "defend" a really good game. Nothing personal.
