I realize that this question is slightly off-topic, but after evaluating the CS Extension Builder v1.5 trial, I'd like to know how much it costs and how to purchase it. Oddly enough, I couldn't seem to find that info anywhere on the Adobe site.
Could some kind soul please point me to the URL with that info?
AFAIK you need to subscribe an Adobe membership such as the Enterprise Developer Program:
Adobe employees will surely give you more details on this topic.
I am glad to hear that you have evaluated CSExtension Builder and thinking to purchase it. There are two ways to do this.
The one is what Davide suggested above. You need to subscribe to the Adobe Enterprise Devloper Program at this link: http://www.adobe.com/devnet/aedp.html
The other option would be to purchase it once it becomes available on the Adobe Store. Until then, you are free to use the fully functional trial version. Check out the Trial FAQ here: http://download.macromedia.com/pub/developer/creativesuite/extension-builder/cs-extension- builder-trial-faq.pdf