About two years ago, I was able to find a web site that had prices for a Visitors Travelcard, and if I recall correctly, it was actually MORE EXPENSIVE than the price for a "normal" travelcard quoted on www.londontransport.co.uk. Since then the Pound has depreciated a bit against the dollar so itīs probably even less of a good idea. But, if you have the chance, you may still want to get a quote for a visitorīs travelcard before you go to the UK - then compare it to the local prices on the London Transport website for a Zone 1 travelcard (I think a visitors card only covers zone 1, where most of the sights are located - regardless, I remember it cost more than the one sold locally for the equivilant number of zones).
Yonatan