I do believe that CF and ASP developers are, on average, cheaper than most PHP developers, but I think it's because PHP has attracted more senior developers from that pool than junior developers.
Where I work a few of the junior programmers are much more comfy in CF, but all the heavy lifting is done in PHP, because CF is just too heavyweight a process to keep up and not versatile enough for some of the things we do in PHP (like building pdfs dynamically, or gif images, or connecting to databases with mediocre odbc connectors but good native connectors etc...)
The real issue of cost of a site isn't in the development, it's in the maintenance, feed and caring. PHP sites seem to need less of this than ASP and CF sites in my experience.