Except that you'll be spending your 2x gold to join those arena runs. You need 300g per week for the 2 extra runs so you can get the free pack. That's pretty much 300g for 3 packs (2 per extra arena run win if you're good enough + 1 from finishing 3) EXCEPT THAT, you get those packs from old sets.
Well played by Blizzard tbh :) They give 2x gold and they get them back again. I'd skip the extra arena runs.