Simple supply v demand economics.
Not many people do the conversion, therefore they can charge more for parts/labor.
It's interesting to note that due to the unreliability, weight and noise of the drum magazines, the Soviets opted to forgo producing drum magazines for the RPK-74 and issued 45 round & 30 round magazines for it. The Soviets did experiment with a polymer 100 round drum magazine, but opted not to produce it. Years later, the Russians developed a 60 round quad stack magazine for the RPK-74M/AK-74M.