This isn't a "what spec is your computer and fps thread"
He asked what is the cheapest.
That being said, WoW is CPU bound in some areas and no matter how good your graphics card is you won't get 60 FPS without a supporting processor.
If you want 60 FPS in all areas with ultra settings, the cheapest you will get is
CPU - i5-2500k
GPU - Nvidia 560ti
You'll need to overclock the CPU to increase the frames in CPU bound areas such as towns and areas with lots of players.
More than fine, get an AMD 7770 or Nvidia 650 or even a 660 and you will be fine, you dont need a 300 dollar video card to play wow well, I still run 45-60 fps on a freaking Phenom 2 quad and a 5770 and that isnt exactly the greatest.
For fps cost, amd is much better. If you look at benchmarks you'll see that we are only talking a few fps difference on most cards however buy what you feel you need. The Radeon HD line be it a 7770 to a 7970 are good cards and do not cost as much as the nvidia counterparts. Same with the cpus, its all preference.
All I will say on my preference, every nvidia card I have ever owned has always failed within 2 years, Im on a 3.5 year old XFX Radeon HD4770 and its still running strong, and yes my settings are on Ultra with a Q6600 Dual Core Intel