Right, well, let's do a little research...
Costs around £210 for a 360 Elite on Amazon. Plus £34.99 for a year of Xbox Live Gold. Hell, let's get the full kit slammed down; PS3 has a ten year lifespan, so does the 360 in theory. That's £314.91, assuming the console has a year of Gold included (which it doesn't, but we'll be kind on the 360).
£524.91 so far.
Just for the record, I can get a 500GB PS3 for £400.09 using Amazon, including a brand new game. Okay, so I need to swap the HDD from the default to the separately-purchased (but included in the above cost) 500GB HDD, but it won't void my warranty. If you try it with the 360, you will void your warranty, if you can get it to work at all.
So, looking at the real costs, even going completely overboard with the PS3, it's cheaper in real terms. I've not even touched any extra features, such as buying a wireless network adaptor (£52.96 for the 360, free for the PS3), or a High Definition Disk Reader (as low as £14.99, but it's the obsolete HD-DVD. PS3 has Blu Ray for free).
Of course, the games are more expensive for the PS3, so in theory it will swing back toward the 360... but games are very hard to quantify by their nature - I've seen MGS4 sold as a loss-leader for £25 just a week or two after release, so that sort of thing can mess up game costing.
Overall, looking at the full life of the console, the 360 doesn't look cheaper at all; it's only cheaper if you focus entirely on the short-term.