Don't forget that the biggest cost in a client is not necessarily the purchasing of the hardware (which is obviously the most visibile cost). Various studies (Gartner, IDC, ...) indicate that a PC that is purchased for $500 (one-time cost) in fact costs somewhere between $1500 and $4500 per year (!) to manage. These hidden costs are mainly into the backend infrastructure supporting these PC's in corporate environments, people managing them, deploying software on them, ... Google for desktop TCO and you'll find plenty of information. Sure, you might disagree with the exact numbers provided by a Gartner /IDC /Forrester but at least it gives an indication.
For thin clients (and desktop virtualization for that matter), this is also where the cost savings are. No serious VDI vendor will tell you that the CapEx (investment in hardware, licenses,...) is cheaper with thin clients and virtual desktops: you need to buy additional licenses, you're going to run desktops on server hardware (ok, 100 at a time on the same box) and then I still didn't start about the licensing galore (Microsoft VECD, Citrix XenDesktop or VMware View or...). The real cost savings are in the fact that it's much easier to manage, and being able to let your very expensive system administators do something else than troubleshooting a desktop (which costs you twice for the end-user downtime and the sysadmin troubleshooting it).
The same goes for thin clients: the up-front investment is larger, but they are very easy to manage (plug into the network and the thing autoconfigures itself, pointing you to your virtual desktop -- which means fewer expensive sysadmin interventions on-site for replacing hardware!), they live longer compared to traditional desktops (these used to have three-year lifecycles whereas thin clients typically have a five-year lifecycle -- roughly speaking you'll need to buy two traditional desktops for one thin client in a 5-year desktop lifespan; I'll concur to the fact that with the economic situation, you'll see prolongued lifetimes for both thin clients & desktops but the idea remains the same, numbers might differ today).
So is the thin client cheaper? In most situations and looking at the total picture, sure it is. Even despite a higher up-front investment. The real problem is not really the price of a thin client but whether your applications and IT environment support thin clients/server based computing (TS/Citrix/VDI).
Sidenote: I work for a consulting firm where I work a lot with VDI & Server Based Computing in general; we strive to be independent as possible (trying to nuance the vendor claims as much as possible for our clients) but that might mean I am a bit biased towards using SBC if it works ;)