Not too sure about pricing as I haven't payed attention to that in a while, but I'll give you my two cents on Crossfire.
I have a HD7990 (two 7970's) which as far as I know is the same as two R9 280x's (they are rebranded 7970's) so my experience should be pretty similar to what you are looking into.
So far Crossfire has worked very well for me, especially since the frame pacing drivers were released. Directx 11.x games work perfectly in my experience, and if you can run them at 60fps (assuming your monitor is 60fps) you will not notice any stuttering. You will only see a performance improvement in this scenario.
However - Some older games running on Directx 9.x will not run as well and you will experience stuttering, even if you are running at 60fps. I'm pretty sure this is because frame pacing has not been implemented for Directx 9.x
TL;DR Will work well. You will see a major performance improvement in new titles. Older games might experience stuttering.
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