The SDK is made up of different parts, and the restrictions (and costs) are different based on which part you are talking about. Take a look at the SDK - which you can download for free - at http://www.adobe.com/devnet/acrobat.html
If you are looking at JavaScript, there are really no restrictions, but there are limitations what you can do with Reader vs. Acrobat (a lot of features are not available with Reader). The same goes for the IAC (or ActiveX) API. The limitations are all documented.
Where you are dealing with restrictions and cost is when it comes to plug-in development. You have to get a license from Adobe to create a Reader plug-in, and what you can do in that plug-in is also severely limited:
http://www.adobe.com/devnet/reader/ik...
Chances are that for the things you want to do, there is no cost associated.