I am working in a company that main tool is .NET.
Last few days I have some will to switch to something open source (learn linux, good web language etc.).
My first thought is PHP as I know its popular.
If there are people here that switch from .NET to PHP world please share was it worth and share some experiences on that journey.
My main concern is that I am planning to run a startup and we all know how sql server + good hosting + IDE cost so I want a life without licences.
Best of luck to you. Did that for about a year and didn’t care for it. Couldn’t find a development environment that suited me (we started off with local PHP in a windows environment, then went to development on VM Linux instances - what a pain) , and there are just enough niggly things that caused me pause. And don’t expect an easy transition. We found that it took us a good bit longer to develop in PHP what would have been quick and easy in .net.
Unless you’re dealing with self-hosting, the costs aren’t that much different for the hosting, at least for any sort of decent hosting. Your real cost would be in your development environment, but even with that, there are “lite” versions which you can run of most items.
Hosting with .net + SQL Server quickly becomes quite expensive, the main expense being the SQL Server license fees.
However, this only become a issue if the website require scaling, if we are talking about a small website hosting prices are pretty similar.
In your case, if you are “at home” working with C# and .net, I would recommend you to keep using it compared to switching to PHP.
The reason being only that you will need a few months to properly switch to PHP and create code at the same quality level that you most probably already do with .net.
Edit:
Forgot to mention that when you start scaling with PHP, you want to move away from some of the “free” stuff as well, mainly because some of the commercial options available means you need less hardware.
Example: Percona instead of basic MySQL, Nginx instead of Apache etc. Please note that both of these have open source versions as well, but in addition they have improved versions with support that require licenses.
[ot]
Unlike Microsoft almighty 
I had one experience with Microsoft products and, I don’t want to hear about them since then. I had to migrate a database from MSSQL to MySql.
After one day of installing a bad version of SQL Server, uninstall, install a good version, get an error that I need PowerShell, Installed it, updated the system, restarted few times, well… got an error that the export feature that I needed was not supported on 64 bits OS.[/ot]