Junkman pretty much summed it up.
How much you pay for a 122 Oxygen (about 8" diameter 4 feet high) depends on who you are, where you are, and what suplier you're dealing with. I picked one up last onth, and for the first time in years asked the price. 11 bucks for the fill of my customer owned tank. A B Prestolite costs me 13 bucks for the fill, 40cf of acetelene.
Now I been buying from that suplier for 40+ years, and my bill is always paid. You could be standing at the counter next to me and would probably pay more for the same gas cash on the counter. The main difference, they know I'll put more money across that counter than you will.
Honestly the gas pumped into the cylinder is probably the least part of what you pay for. Small bottles and large bottles both cost as much labor to fill, and the insurance cost is equal too. Then add in the half million dollars the dealer just spent on fill equipment to do Oxygen, Acetelene, Argon and Helium. He ain't eating that cost. Then there is the BIGGY, insurance. Every bottle going out the door pays part of the insurance cost. The damn insurace company is making more $$$ per bottle than the dealer is, and has no investment in the business.