Well, right behind the computer, I've got a stack of dozens of CDs, worth hundreds of pounds (to replace, at least). Above them, there is a collection of RPG books, which each one cost me about Â£20, so around Â£700-Â£1,000 there. Then there's the literature collection, the electronic equipment (TV, PS3, Stereo, iPod and my mobile phone) and of course the furniture that it all sits upon.
I've got a collection of ties, which cost me a pretty penny, plus my cufflinks which are becoming a habit. Finally, in answer to your question, the most expensive thing that I own in terms of monetary value would be me.