Packing and licensing an API

I've started and launched a small SaaS application. It offers a simple API that allows developers to create and manage passes for Apple's Passbook.

Part of the offering is an implementation of the Apple defined API that passes use to register themselves and to fetch updates. It isn't a very difficult API to implement, but the storage and management of the data behind it is reasonably complex.

I've had two developers ask about licensing this component of my application as they would rather host it themselves, rather than pay the fees I would charge. Whilst this isn't ideal, I'd rather get something for my efforts.

As my service is new, they are concerned that it may stop be available in the future. To offset the risk, they would like to integrate my code into their solutions.

I am willing to license my code to them, but I'm unsure how to place a value on the source code.

Would anyone be able to offer me advice on how to proceed with licensing the code?