My dear friend:
I don't see why it should be expensive to use ASP. What I think you really mean is SQL SERVER is really expensive (assuming you are using SQL Server as a backend, because ASP IS FREE). ASP is standard in any Windows Server.
In any event, PHP is just like ASP, it is a filter for HTML requests, so it can be installed in most web servers. You dont need to worry about that. All you have to do is find a hosting company that supports PHP. AS SIMPLE AS THAT.
By the way, the reason why MS SQL Server is more expensive is because MS SQL Server is Microsoft, and MySQL Server (used widely with PHP) is FREE, yep, thats right, open source.
Good Luck!