Hi There,
As in you project description, you only need conversion from your existing project (C#/MS SQL Server) into PHP / mySQL. It will be better if I can see current aspx's site structure or you can give me more information about the nature of your aspx site, so I can give you better estimation. If there is no special widget used, the migration can be performed easier.
If your aspx's site does complex data presentation, we probably need to renegotiate project time.