Hi,
I propose to create an adapter API in .NET (WCF or classic asmx, or both, your choice) that calls your PHP API on the backend. This way there will be no need for duplicating and maintaining logic in two or more languages.
I can host the implementation on my own servers and provide you with access URL. If you found its performance satisfactory I will hand over the source code to you.
I can also do the actual re-implementation in .NET for you but will need more details about how the logic is implemented in PHP and will definitely cost more than $30.
Please feel free to contact me through email.
Thank you,
Arash