Supposing you get it working in your current configuration, you may have problems with screen tearing if you run a grouped eyefinity display across displays attached to multiple adapter types, and this is across all AMD Eyefinity setups regardless of card, so that's something you may want to consider before parting with anymore cash
I have this issue, but have 7950 card and a AOC q2963pm display with DP daisy chaining capability.
If, in your setup you want to run three screens without screen tearing, I think (although I've yet to get confirmation of this myself) you want to run them all off the same adapter type, which you can do using one of your displayports and a MST hub such as this one, and then run three DP to whatever adapters to your monitors, although I don't know if these will have to be passive or active given you're going through a MST hub.
At the moment with my setup I'm very much in the 'theory' stage of getting another AOC 1080p monitor using DP, and then using my DP out on my 2560x1080 monitor to my existing HDMI 1080p monitor.... I have a 4480x1080 dual monitor setup at the moment but the screen tearing is very distracting,
Although like you mention, it's all cost.