I get asked often how much SharePoint Designer costs, and I never have the answer. I guess I've always been spoiled with Subscriptions through MSDN, TechNet, etc.. The SharePoint team published a post on licensing for SPD: http://blogs.msdn.com/sharepointdesigner/archive/2008/11/14/spd-licensing-101.aspx
The software is licensed per desktop, and has an estimated retail price of $299.95. You can upgrade for $159.95 from different applications (see referenced post).
You can purchase it online.