It has all the multiplayer from Halo 3, including $17.49 (1400 MSP) worth of multiplayer maps. Then on top of that it has a new mode, some new maps, and a 6-hour campaign. The $60 price is fairly reasonable. I tend to think of it as all the extra multiplayer stuff not in Halo 3 proper as $20, and $40 for a budget sized Halo single player campaign. Your value on such things will of course vary, but for me it works. I'm not really interested at all in having the game, but I bought it because I like playing with my friends who are all hot and bothered for it.
Now, if you are only interested in the new SP campaign or already paid for the extra H3 maps then... well MS says f*** you.