Maybe already considered this, but you can often find used expedition parkas for a fraction of the cost.  I've seen lots of them here, garage sales and used gear shops.  Folks spend the $500+, use it for a few weeks on the big mt, then no longer need it.  Just an idea... 
  
That said, I did fine with a cheapo $100 Burlington coat factory gangsta puffy.  We did the north side in April/May with temps as low as -60F. I just made do by adding more layers.  I'd really regretted not buying the -40 sleeping bag, however.   Folks were climbing high with animal skins back in the day and seemed to do well. But yeah, get the best you can find, whether it is new or used...