It's not quite as simple as that. The 360 lacked a lot of features that were included with the PS3.
With the shocking reliability, disc gouging and unbearable noise the 360 was the worst piece of consumer electronics I have ever owned...and I've owned an Alba clock radio! Even the Pro version lacked HDMI, wifi and if I recall - the ability to recharge the batteries in the controller. They've fixed some of that now, but it would require purchasing an entire new console. A few years of Xbox live certainly adds up too. The 360 may have been cheaper, but it was a lesser product.
The launch PS3 may have been £400, but that was all I needed to spend. Wireless controllers with charging cable, multiplayer, HDMI, blu-ray, wifi were all included. Plus the console I bought at launch is still working...
You get what you pay for.