In an interview with Polygon, Microsoft Studios head Phil Spencer was asked if Halo 4 was the most expensive game Microsoft's ever produced, he replied, "Absolutely" and "nothing's even close."
To give an idea of the money that went into previous Halo titles, Halo 3 is believed to have cost $60M to deliver, including a $30M advertising budget. Meanwhile, Halo 3: ODST made $170M in its first 24 hours of release.
According to Spencer, the entire Halo franchise is worth a cool $3 billion.
Halo 4 will be released in less than two weeks for the Xbox 360, and it'll be the first game in the series not developed by Bungie. In short, a $3 billion franchise is riding on 343 Industries' first effort. No pressure, guys.