Yes, the Sage 50 Accounts SDK is required if you want to integrate your accounting data to Sage... And yes, it costs money: about £1500.
For this you get full access to the development support team (who are really helpful and know what they're talking about), the SDK DLLs, comprehensive SDK docs and the activation keys in order for you to start using the SDKs with Sage.
Note - the example codes are written in C# and VB.Net.