PHP is more expensive to code than .NET is not true at all
I actually have found just the opposite--
PHP tends to be much less expensive to code in, than.NET--at least for my organization. And because MS doesnt tend to scale as well, and usually requires more hardware, you have those additional costs. And lets not even get into admin time, or security issues--
You also have to take into account the ongoing licensing fees from MS. PHP coders may be somewhat more expensive, but its usually not by much. Again, situations differ from place to place. If you are working with a vendor who is strictly MS, and you have no choice but to work with that vendor, then your hands are obviously tied. However, if you have leeway, I would seriously consider a PHP approach--