Most GPU mining discussions focus on how long you would have to mine to "ROI" your card, or get the $ value of the new card back in mining revenue.
But I've found you can easily sell top end GPUs for half their value new a year out. If you can manage that (shouldn't be hard), then your cards have ROI'ed when they have returned half their retail price.
So if your Rx 280s are going to return their cost in a year, if you mine for 12 months and then sell your cards you realize a 50% net profit, not breakeven. So a year in your $2.5k rig returned about $2.5k in mined coins and $1k in resale, assuming you sell the cards only. So your breakeven wasn't at a year, you only had to mine for 6-7 months to recoup your REAL investment--not the retail price of the rig, but only the cost of the PSU etc and the 50% of the cards value that evaporated when you purchased it.
Hope this encourages some small scale/hobby miners out there. Got to keep the decentralized mining strong.