I was wondering the same too but still cant figure out the best price to performance.
Maybe I'll get a 6950 for 14k and unlock it into a 6970(480khash)
6950s are great. but i'd rather put my btc in
https://cex.io or
http://pbmining.com and buy some Ghash. you would be getting the same income as mining scrypt.
so after a couple of months you could buy yourself a r9-280x from the income you accumalate from your scrypt-mining with your current cards and cloud-mining.
if you follow this advise, there are a few things you should consider.
cex.io will cost more than double for a Ghash than pbmining. but at cex.io you'll get instant pay out after block confirmation and you could buy additional Ghash with that on the go and also you have the advantage of selling your Ghash if you need Btc.
At pbmining it will only cost 0.0089BTC for 1 Ghash. but you'll be paid only at every Sunday and also you could buy only in multiples od 1Gh\s, but whereas at cex.io you could buy fractions of a Ghash.
Believe me this will seriously work