As a run-of-the-mill business application developer, one usually pays for presentation layer software from DevExpress/Infragistics and so forth.
Money usually goes to attaining the latest and greatest Office mimicking UI, and your developers ‘thrash out’ the necessary business logic. Rudi Grobler has just posted a simply excellent example of creating an Office 2007 Navigation Pane in WPF, using just the standard WPF tab control. Usually one  would have to spend $200-$400 (without the source code) from – to choose two random presentation vendors –  http://www.actiprosoftware.com/Products/DotNet/WPF/Navigation/Default.aspx
or http://www.devcomponents.com/.
The real beauty here is that one can skin the complete application using the same resource dictionary, which is super-duper-powerful! This allows for a uniform ‘look and feel’ across the application, normally an absolute pain to implement with windows forms.
When you also take into account that Scott Gu recently announced that a major update to WPF is imminent (summer 2008), and that WPF will have a new Ribbon Control and DataGridView out-of-the-box, a personal proclivity is developing toward WPF.

This summers update will address key problems I have highlighted in the past with WPF, primarily poor performance with list type data, and an unavailability of third party controls. If you also ‘factor in’ .NET cold and warm start up times being significantly improved, the WPF ‘stack’ is certainly gathering momentum.