A guestbook page signed by Houdini's assistant Franz Kukol, complete with a nice photo of Kukol that I've never seen, is currently up for auction on eBay. This is from the week of May 24, 1909, when Houdini was performing at the Portsmouth Hippodrome in Southsea, Hampshire, UK.
The auction starts at £999 ($1,622) with £82 ($133) shipping and insurance. Top dollar for a Kukol item to be sure (heck, it would be top dollar for a Houdini item). I'll be curious to see if it sells.