it can use 400-450 watts if you overclock it

for bitcoin mining, you can get close to 300mhash on a 7790  for about 70 watts

unsure what the current prices are, but newegg used to have this sapphire 7790 on rebate for $110 over and over.  also a powercolor for $115.  after you sold the games, it'd be around $85 and $90 respectively (they were worth more back then, like a month ago).  

now, obv this requires some of your time and effort (10 minutes to do all the crap for the rebate and maybe 10 minutes to list the gaming codes on ebay and another 5 minutes of BSing with the person that buys em?)...   but throwing that aside, you could just not even open the box for that card and sell it as 'brand new' on ebay and make about a 20% profit after fees (assuming you have a shop or w/e).

or you could mine with it for a month or two then sell it.  not sure which way is better.  i chose the mining route, but my elec is 6.8c

(another ed: hmm.  i wonder if there will be lots of 'clearance' deals on radeons here soon)