Well, he was specifically interested in BF3 performance.
That is a fair point, though if you read again, it looks like it was just an example. It's obviously a very subjective thing - how do you decide how much monetary value to apply to a percentage performance gain? Especially when value doesn't scale properly with price - at the high end, 10% will set you back £100. At the low end, £10.
£50 is a fair bit of money though (it's a little over $75 for American readers). Some people might think that's a reasonable price premium for 10% gain, but I'd say the GTX660 is a smarter use of money.