php or .net for startup

Question

I want to start my own business. I have 3+ year of expericence as .net developer.

but .net platform is very expensive to start work. I have also knowledge about php.

but in php i need to polish my knowledge more. php is open source and free platform.

I have very confuse wich platform i choose for development. I have knowledge of .net.

but .net is expensive and php is open source but i need to prepare from starting.

it take lot's of time. Kindly suggest.