Deal Or No Deal In Shares?

You are on a game show where there are 10 boxes, nine of which contain nothing and one of which contains £100,000. You can pick any one. Having done so, but before opening it, they make you an offer of £5,000 to withdraw from the game. What is the correct strategy: take the offer or open your box? This is a simplified version of the game show Deal or No Deal, which I saw recently at a friend's place because she is a great fan of it. One thing I noticed is that the offers were always below the average per box, at least when I watched it.