Hiya TL - hardbacks often cost a different amount because of the timing and the seller. If the book is by a popular author and is on a big general release (say a Pratchett or a Feist) then the price seems to start at $25 US of the release date and rise to $30 afer about 3 months if the hardbacks sold well (it usually stays down if it looks like the shop will get stuck with a lot of unsold copies). If the book is an American release well the price is considerably higher - $35 US - $50 US. For these I have to go through the specialty fantasy book stores.
Why do I opt for buying the American release of certain books? Mainly as I said before - there are some books I don't like waiting for. The WOT series is indicative of this. The options are to buy a hardback on the American release date for $45 US, or wait another 3-6 months for a paperback to appear in the general bookshops. I am yet to see a hardback release in the general bookshops of a WOT book.
I either lack patience
, or am a wastrel - I haven't decided which it is yet
.