I priced out the options for a beginner that was getting a hive for Christmas, and all-medium was about 30% higher cost than deeps.
There was question a while back about why commercial beeks didn't use all mediums, that answers the question from the pocket book.
That aside, I made my choice of all mediums based on several factors and cost was one of the lesser considered items.
1. I'm a hobbiest.
2. I have health issues, so what configuration makes the most sense for me (Weight, weight, weight).
3. I want to be able to do splits, conbines, super, hive balancing etc, with the greatest of ease.
4. I can build some of my own equipment.
5. What other types or style of equipment do I want (ie SBB, migratory or telescopic tops, queen excluders, etc).
6. Do I need Grade A equipment.
7. Will factory 2nds get me what I need at a price I can afford with a sufficient length of use involved.
I recommend people make such an evaluation before they make their 1st purchase. Once they've made their decission they can buy accordingly.
If they decide they want to do something different latter, they can.
For me, the extra cost associated with the medium supers is offset because of health issues, bulk purchase of 2nds, in addition to making my own stuff.