You have to consider the total cost. If the Apple hardware lasts longer, requires less maintenance and makes the developers more productive, it may end up very well paying for itself. My wife uses a Mac and just the time I didn't spend fixing, cleaning and disinfecting a Windows install more than paid for the price difference between her MBP and a cheap Dell. Compare it against a similarly well built machine and things look even worse for the PC. Her current Mac (a 13" i7 MBP) survived a car crash in early March (the machined body is very slightly warped and it'll probably have to make a visit to an Apple dealer for that). Her previous one, a 2006 white MB, sits on my desk as my secondary computer and is our main source of ambient music. It had an aesthetic problem, which was fixed by Apple for free (because other parts had to be replaced due to a recall) last March. It replaced a (still working flawlessly) 1998 iMac in that function. You can say anything about those machines except that they aren't built to last and that their customer service isn't stellar.
If I had to use my Dell with Windows, I know I would be far less productive than I am under Linux. I know because I tried (from 2008 to 2010).
Unless you are developing for Windows (something that pretty much implies you are running Visual Studio), I wouldn't advise you to use Windows as your development platform. And, if you aren't, it's only natural that testing on Windows incurs an extra cost.
Having said that, it shouldn't be that complicated to set up Windows VMs to run automated Selenium-driven tests and plug those into Jenkins.
If you test manually, you are doing it wrong.
If MS could offer the same, no problem. Those two days of struggling and the repeat performances to come are way, way more expensive than buying a Mac. My time and the time of my devs is way more valuable than a Mac Mini.
My company has to do that and we spend countless hours on IE specific tasks.
It works on IE10? great! I don't know if it will on IE11. Or if microsoft will find another way to break open standarts.
If we had a choice of not supporting IE, we'd do it.
Not because of Mac, not because of Linux. Because of IE and nothing else.