Joomla Web Service Module Development

I require the development of a custom PHP module that manifests as a web service with the use cases:\\r\\n1) Create user: When called from an external asp.net application it will create a registered user on Joomla or return user info if user exists or error\\r\\n2) Validate user: When called it will return the user information or that the user does not exist or error code\\r\\n3) Activate user: When called it will activate and blocked user etc.\\r\\n4) Inactivate user: When called it will inactivate a user or return error (e.g. user does not exist, user inactive, connection timed out etc.)\\r\\n\\r\\nIf we can get this working I have further Joomla integration requirements that I would like build via Joomla modules and using we services or a better approach if you can suggest one. If I am smoking my socks on the project budget let me know what you think it should cost.