With PHP, there's an abundance of developers who can write code in PHP. They may or may not write the best code, but the barrier to entry is much smaller. So, given that there are more of them, you can get PHP developers at a bargain basement price. My advice though; don't go for the bargain. The technical debt will be more than you save. This is not because of the language. At. All.
With ASP.NET, you're going to pay more. The platform costs money, so the developers you're likely to attract are those currently working at medium, to large size corporations or small development firms with big customers. There's also a lot more to know when it comes to .NET, i.e. the page request model, MVC, threads, etc. Also, you didn't say if you were looking for C# or VB.NET developers. VB.NET is likely going to be cheaper to find someone to onboard, but for some reason, the C# developers tend to know more than just VB. It all depends on the candidate.
ColdFusion, that's the skill that got me my current job. If you're going this route, be prepared to train someone or search high and low for a very long time. ColdFusion is great, but its popularity is dwindling. Here at the office, we're converting all of those applications over to ASP.NET 2.0 using VB.NET. There are a few shops still using CF, but the talent pool is rather small. Training someone would be your best bet, but you'll need people who can train the neophyte CF programmers. If you're using Mach or any of the other frameworks, the same applies. The more things you require them to know about CF, the more you're going to pay.
If I were you, I wouldn't bother choosing your technology using your budget. Find the talent, then see what it costs to hire them. The best developers will be polyglots, multi-lingual, and agile enough to pick up new technologies.
Whatever you do, do not, under any circumstances, list " W years of XYZ required" in your job posting. That's a dead ringer that you're not looking in the right place.
Good luck with your hunt!
If you're looking for good, stable CF developers -- check the user groups and talk to the Adobe Evangelists. They have a nice job board. :)