The old Mac Pro with a modern PC GPU.
If 2x 4K monitor is all you need regardless the processing power. Even the Mac Pro 1,1, can do that with a current PCIe graphic card (e.g. 750Ti). This setup can give you dual 4K output with less than $150 (of course, it will cost you some time to buy the cheap stuff on eBay).
A more make sense combination is something like the Mac Pro 4,1 with the Maxwell GPU, e.g. GTX 950.