It requires mercury-198 as an input. The only quote I could find for mercury-198 is about $15K per milligram.
So now we just need to figure out how to make mercury-198 cheaply.
That's even better! We can turn gold into mercury-198 and sell that. Gold is dirt cheap at $0.11 per mg. We're rich!
There are 7 stable isotopes of mercury, and mercury-198 makes up ~10% of naturally occurring mercury. The paper covers a lot of ground here, see section 5.2.2 "Mercury Isotope Separation", where they are shooting for $2.4/kg.
It's 10% of natural mercury. you're looking into separating it cheaply instead, or at least hope the other naturally occuring isotopes don't cause too many problems.
It's the second lightest natural isotope of mercury, comprising about 10% of natural mercury:
https://en.wikipedia.org/wiki/Isotopes_of_mercury
Since mercury forms vapor so easily, it should be easily enriched in gas centrifuges like uranium (more easily, actually, since the starting isotopic abundance is higher and the chemistry is simpler). The high price of purified mercury-198 at present is probably due to it being a scientific curiosity with no industrial demand.