Depends on what you get... if you focus on ferengi boxes, 1 D'kora and 10 ferengi duty officer packs will get your money back - on the other hand, 30 shield packs and 10 mirror universe recon science vessels will not get you much more than another key.
When I open old boxes, I prefer the Cardassian ones, because those Gamma Quadrant Elite doff packs are nice (a couple of the possible doffs are worth a ton of EC, or great to use if you don't want to sell them).
I would stick to Cardassian and Ferengi boxes to be safer on your returns. The Ferengi seem to yield the highest Lobi cap, but the Cardassian seem to yield higher EC items. If you're feeling lucky, you could try the Temporal and hope and pray for MKXII Polarized Disruptor Turrets, as they can fetch a nice chunk of EC.
I would say play it safer and stick with the Ferengi or Cardassian.