The "cost" of development is (roughly) $100,000 per developer-year, plus the costs of licenced technology (and in the future the licenced game assets) ... Gears of War only cost $10 Million to make because it was (roughly) an 8 hour game with a reasonably focused multiplayer component ... They could have (easily) spent $20 or $30 Million to produce a moderately long (16-24 hours) single player campaign with the multiplayer options that were available in the previous generation.