I might have this wrong but I think that Adobe forms are part of the commercial backend Adobe products, so there would be licensing fees involved. With flex you can develop RIA's that can have form based content that links into any backend you want to and not wrry about additional backend costs..
I dont think there is such a product called "Adobe Forms" I know there is Windows Forms, and also
PDF Forms but first time I hear "Adobe Forms"
I assumed the original poster was refering to the Adobe Livecycle Forms module.