Anything that says medical on it will automatically be higher priced. (Thank God for lawyers, malpractice insurance, and folks that try to make a living by suing others) I would assume that as long as you use a hepa filter, you'll be OK with the welders O2.
I ended up with a medical tank for free so it's tough for me to argue either way. You will find quite a few on Craigslist from time to time but I suspect it will be difficult to get a fill unless you know an EMT or nurse (my sister is an RN).