I agree with the gist of your statement, but a if an Air is good enough for iOS development (and there are a few blog posts out there that say it is), then a baseline Mini will be fine.
It's amazing how much that updates compile speeds.
Buying a mac mini to do iOS development is buying pain and a computer you're just going to have to replace.
ifixit.com seems to indicate otherwise: http://www.ifixit.com/Guide/Repair/Installing-Mac-mini-Model...
I don't know of anyone putting that forth as the low cost solution (nor is that sort of change out easy for someone who's really terrified to break their $700 computer and not used to fiddling with that stuff).