Adding a Wi-Fi adapter to an Xbox 360 will cost you either way, but such a purchase depends on whether you actually require Wi-Fi, or if your setup will allow easy cable connection. Additionally, the press release claims that you will lose living room space with the Xbox 360 console when you start to buy add-ons, thus you, "add clutter to the entertainment center."
The HD DVD add-on might apply here, but that is now obsolete. The Wi-Fi add-on is very small and clips right on to the back of the Xbox 360 console. All other peripherals from controllers to cables are similar in both systems. The Wii did not escape criticism either, with the press release claiming, "the Wii's lack of enhanced features comes at the expense of a comprehensive entertainment solution."
The truth is really that whether or not the Xbox 360 will turn out more expensive than a PS3 console depends entirely on the customer that buys it, and whether they need or want a Wi-Fi add-on, a Gold membership, a hard drive (if they didn't get a model with one already, as the "choice" is the reason for the Arcade model's existence) or a defunct HD DVD add-on.