280x's are sweet but they've been very rare to find the last month and are priced over MSRP by about $100 right now. It is much more efficient right now to buy standard R9 270's (non-x versions) then overclock them.

I have 8x standard 270's going right now running at 425-440k each. Standard 270's also only take 1x PCIE 6pin power plug as well. Cost was 179.99 per card and all of them included BF4 key tokens which were able to be sold for about $25-30 each.