As a hobbyist myself as well, I'm paying about $35 for 60CuFt refills, on customer owned cylinders. Same price, for a refill, or exchange.
A friend of mine with 80CuFt tanks, only pays a buck or two more.
Pricing in my scale is definitely based mostly on cylinder handling. The most extreme example of this, is between an MC, and B acetylene exchange, where the B is four times the size, but only about 10% more to have filled.
Helium, now that's a different story. Every time I get a refill these past few years, I was either told that I was lucky, and that a price increase would be happening in the next few days, or that I just missed out on the lower price by a few days. Yes, large volumes are better deals, but helium is just getting ridiculous.