I'm researching what I need to setup 3 or 4 monitors on a XFX HD 6970. I plan to use a projector in hdmi and two additional 23" monitors (flight sim). I believe that you must use at least one of the mini display ports to enable more than two monitors in eyefinity. Since I don't think any monitors come with mini display ports, it appears that AMD has conned everyone into needing this 90$ adaptor.
I know you don't need a dual dvi link adapter to use a mini display port adapter, you can find cheaper adapters that will work, I personally don't know which ones specifically nor intend to do the research for you.