DanielRavenNest writes: "It takes 2-3 years for the tug to do the return to cislunar space (near the Moon's orbit)."
That sounds like a round trip is going to be 4-6 years plus the dwell times at the ends (which may not be significant in low-hanging-fruit scenarios).
What is the rate of interest you're using for the amortization? What does the tug cost up front? What is the price charged for the hydrocarbons and oxygen produced? What price elasticity of demand are you using?
PS: Thanks for not being "not even wrong".