I linked my source at the bottom of the table where it shows the price. They are based on the SIM/Contract Free versions of the phones from Expansys to give a real value of the phone as available to anyone. I found them by searching for "iphone 3g s" and "htc desire" on their site, and looking at the first one that doesn't come with a load of accessories or extras, in an attempt to give the most accurate cost of the phone on its own without subsidisation from a network or having an inflated cost due to extras.
iPhone 3G S = ?799.99: http://www.expansys.com/d.aspx?i=183742
HTC Desire = ?439.99: http://www.expansys.com/d.aspx?i=196001
So, the HTC Desire is 55% of the price of the iPhone 3G S, which is pretty close to "around half the price".
Expansys was the only site I looked at, as I use it frequently. If I made a mistake I am happy to correct it.
Edit: OK, on researching that further, Expansys sorts by the most expensive item first, so the above is a link to the black iPhone, which is ?100 more than the white one which is currently ?694.99. Still, that leaves the HTC Desire at 63% of the cost of an iPhone 3G S, which is pretty close to "around half the price".
Edit 2: And to be really fair, the iPhone 3G S linked is 32GB, and the HTC Desire comes with a 2GB SD card... so add ?70 for a 32GB micro SD and you get ~?500 versus ~?700. Not "around half the price" any more, but still a damn site cheaper. I updated the blog entry to reflect all of this. Thanks for giving me a nudge to double check those figures.