Would if be worthwhile learning PHP or would ASP.NET be more useful to a web designer?
Learning ASP.Net isn’t as expensive as running it for a live web site: the development environment can be had for free (with the Visual Studio “express” editions, http://msdn.microsoft.com/vstudio/express/), and IIS is included with both Windows XP and Windows 2003, so the operating system which you probably already have is the only software cost which is truly required. Mr. Woods is correct that to actually run an ASP.Net web site requires licensing fees, but if you’re just learning then presumably you’re expecting your employer to pay that stuff when you get a job. I will say that in my experience, ASP.Net is a very powerful framework. I also prefer open source, so its a bit of a tossup. One thing to consider if you’re looking at it as a profession is to just pick one and start learning — many of the skills will translate to other platforms. HTML, CSS, and Javascript skills are going to useful in both environments, and many of the programming idioms will also translate well.