Which refrigerant is the most expensive?

Which refrigerant is the most expensive changes over time due to supply, demand, and phase-out regulations, but generally the most expensive are older phased-out refrigerants that are no longer produced and low volume specialty refrigerants. As of recent years, R-22, also known as Freon, has become one of the most expensive common refrigerants because its production and import were banned in the US from 2020 under the Montreal Protocol, so only reclaimed or stockpiled supplies are available, causing prices to rise dramatically to several hundred dollars per cylinder. R-12 is even more expensive and rare due to earlier phase-out. Among current production refrigerants, low GWP HFOs like R-1234yf used in automotive AC is very expensive compared to R-134a due to complex manufacturing and patent protection, costing several times more. Specialty blends like R-1233zd and R-1234ze for chillers are also costly. Natural refrigerants like propane R-290 and ammonia are actually cheaper, but equipment costs higher. Price also depends on region, purity, and cylinder size. So historically banned CFCs are most expensive, currently HFOs like R-1234yf are expensive among new options, while R-22 remains pricey due to scarcity for servicing old systems.