I've started to build a mining rig again, this with 7950's, 7970's and 280x since they're very cheap in Sweden.
I'm buying 7950's at $30, 7970's at $40 and 280x at $50. I find this very, very cheap. Just 2 years ago I sold 280x at $200 and 7950 at $150.
Of course, these graphic cards aren't the best anymore but they're still very good for mining.

Old vs New

Right now it's more profitable to buy these than to buy new cards, it will take me maybe 40 days to get my money back. Excluding price of electrics which I myself don't pay.

Old

Hashrates

• 7950 - 200sol/s
• 7970 - 220 sol/s
• 280x - 230 sol/s

Time of ROI:

• 34 days on 7950 ($30 second hand)
• 39 days on 7970 ($40 second hand)
• 46 days on 280x ($50 second hand)
• 41 days with all three ($120)

New

Hashrates

• RX 480 - 260 sol/s
• Fury X - 360 sol/s
• RX 470 - 250 sol/s

Time of ROI:

• 155 days on RX 480 ($180 second hand)
• 135 days on RX 470 ($150 second hand)
• 250 days on Fury X ($400 second hand)

All hash rates are based on recent posts in the Claymore Zcash Miner-thread on Bitcointalk.
All prices are based on prices on second hand value in Sweden right now, except the Fury X which I took from Ebay.

Of course, in this "study" I didn't add the price of motherboards, CPU, PSU etc to get the $/hash but the price is not so high that it's noticeable when the graphic cards are $100-350 more expensive.