There are 4.6 million non-white people living in the UK (2001 figures). So:
4,600,000 people x £50,000 = £230,000,000,000
That's £230 Billion!! To put that in context total UK spending is around £620 billion per year and the cost (just in incentives) is put at well over 2 times the total NHS/Health which is £110 Billion per year.
But that's just the incentive cost. Yes, you would not need to pay any benefits etc for these people any more, but nor would you have millions of working people no longer paying taxes into the pot. Plus they would be taking billions of capital with them out of the UK. Another blogger put this at £300 Billion - which is also an understatement as you need to cost in the administration of the scheme moving 4.6 million around, which would cost millions more and the loss to the economy. In short this is the policy a bunch of racist-fuckwits would come up with having a piss-up in the pub.