To develop a plugin for Acrobat there is no cost or license required. Just download the SDK and have fun! Any plugin you develop, however, CANNOT be used in Reader - only in Acrobat.
If you wish to develop a plugin for Reader, you will need to obtain a license (at some cost) from Adobe. Not all requests are granted, so even if you are willing to pay, you may still not be able to do so. If you obtain the license, then plugins you develop for Reader will also work in Acrobat.