As Pau stated in a reaction in this blog entry, current development of applications using Flex is expensive. I have no idea how expensive, I never used the current Flex system, but if I may believe the many stories on the internet it is very expensive (everything is relative in this case however;-)). I heard a price tag of $10.000, but hey, thats quite normal in server software licenses. By the way, there is also a free license for non-commercial/nin-institutional applications and for bloggers who want to showcase on their blog. Have a look at: http://www.macromedia.com/software/flex/productinfo/faq/#section-8
How about the new Flex 2 system? If you read this story by David Wadhwani, VP of Product Development for Macromedia Flex, he writes:
If you're familiar with Flex 1.0, it's very important to recognize that Flex 2 is far more than just a new release. It represents a major milestone in the evolution of the Flex technology and a continued evolution in Macromedia development processes. From a technical standpoint, Flex 2 introduces capabilities that enable developers to build an entirely new class of Rich Internet Applications, ushering in a new generation of RIAs. At the same time, we're opening up Flex development to a much broader group of developers by re-introducing Flex Builder, which has been built from the ground up on the Eclipse open-source IDE framework and now includes the Flex Framework and the compiler. That means that Flex applications can be deployed as a stand-alone option by placing a compiled SWF file on any web server or in conjunction with Flex Enterprise Services 2.
This means that no server component is needed is you need webservice connections only.
There will be multiple product in the Flex product line. One of them is Macromedia Flex Framework 2. This is a framework that build on top of the foundation of Flash 8.5. It is a client side framework. If you only utilize this, the price must be low. How low? Probably in the same range as VS.NET is my guess.
Also with the current Flex system it is possible to build stand-alone Flash applications if I read this blog post correctly.