You can realistically spend whatever you want. Nintendo spent $25-30 million on Mario 64. And for marketing, the sky's the limit. THQ indicated the cost of 360/PS3 ports is low double digit percentage of initial cost. Short of a marketing/publishing arrangement or direct payout, this makes exclusives unappealing, unless you expect your audience to be primarily Japanese. With that in mind, I'm in agreement with your theory that it makes a lot more sense to develop for both PS3 and Xbox 360 and forego Wii in many cases.