We're going to pretend we're either a family of four or an apartment that wants a system loaded up with four controllers. We'll assume you want to be able to recharge your controllers rather than wasting money on disposable batteries, and we'll pick the model of each console that we'd recommend to a family member asking us for advice. We'll also factor in the cost of 2 games, because nobody buys a console without a game.
So what's the true cost of going out and buying a system today? Let's find out.
Cost of console: $250
Cost of 3 extra controllers: $36 x 3 = $108
Cost of 3 Nunchuks: $18 x 3 = $54
Cost of 4 Wii Motion Pluses: $20 x 4 = $80
Cost of charging station w/ 4 rechargeable batteries: $49
Cost of 2 games: $100
SD card: $12 (8GB)
Cost of console: $300 (60GB)
Cost of 3 extra controllers: $37 x 3 = $111
Cost of 2 rechargeable batteries: $12 x 2 = $24
Cost of recharging station w/ 2 rechargeable batteries: $30
Cost of 2 games: $120
Cost of Xbox Live membership: $43 (13 months)
As you can see, the prices of all three consoles are incredibly close, and you can obviously fiddle with these configurations to change them. If you only want 2 controllers and are planning to only play Wii Sports, the Wii is still the cheapest, but a fully-loaded Xbox 360 with a year of Live is actually cheaper than the fully-loaded Wii (although it's the only console without WiFi, so add $80 if you really need that). And the PS3 is surprisingly close in price, thanks to its controllers having built-in batteries and its free online service. The Nintendo controller scheme really rockets it up in price, and it's pretty amazing that a Wii with 4 controllers is only $24 less than a PS3 with 4 controllers.
So really, it comes down to what you want in a console. But it's surprising to see that, in real-world prices, all three systems will set you back about the same amount, with the Xbox 360 being the cheapest of them all even when you factor in a monthly fee for online gaming.