If I could get a job doing X or Y, and be happy at either, but X is $20,000 more than Y, I'd love to know that so I can work on those opportunities more.
More narrowly focused skill-sets on the other hand will have variation...for example, developers that solely want to work on the front-end (e.g. backbone) are going to make 20K-40K+ less then their full-stack counterparts.
I had to read this sentence about 5 times before I realized that "backbone" was probably the name of a UI library.
I can't help but wonder if I am sabotaging myself by spending time on ASP instead of PHP, and would love some hard numbers to help me make a more informed decision.
Another bit of advice would be to work in a position where you have your hand in all aspects of a project. That will help you decide if you want to focus on a particular subset of application development (i.e. back-end, front-end, UX), or if you want to move forward as more of a generalist. This decision will impact your future type of employment. Concentrating on one particular aspect of development will lead you towards larger companies and/or larger development teams. Being a generalist will lead you towards smaller companies or start-ups with smaller or no teams at all, as well as possibilities outside of normal technology companies (i.e. I am interviewing with a law-firm next week that needs a senior developer to create internal tools). I'm of course speaking only from my experience and making generalizations on that basis alone.
PHP is a great first step on the indie dev path. You'll work in smaller shops, pay can fluctuate wildly, but at least you'll be a profit center. It takes a bit more hustle to manage your career but you'll be better positioned to hang out your own shingle later if you're interested in doing so.
A few years of enterprise work can be a fine starting point for transition into more indie dev, but you'll have to be honest with yourself about the trajectory you're on from one year to the next.
Now that I'm graduating I received several job offers, most of them involved working with PHP again. However, I received two offers from two relatively large companies, one as an ASP.Net developer and the other as a Java EE developer. The salaries for these two jobs were quite a bit higher than the others and included great benefits too.
Again, I'm only a graduate so can't really give any solid answer as to whether you're sabotaging yourself by working with ASP.Net, though I doubt you are. Most decent employers will probably be more interested in ensuring you can program rather than focusing on what specific language you've been using recently. If they know you can program, they can probably assume you'll be able to apply that knowledge to a new language.
Also, salaries for experienced F#/C# developers are somewhat higher than those of experienced C# developers. Do some research if you don't believe me.