For your reference, records indicate that
Thomas Koenig wrote:

An older version of a Falcon 9 reportedly had 488 tons of total
fuel, 147 of it RP-1 (a modified kerosene). Liquid oxygen is
quite cheap, and if we give RP-1 a cost of 1 dollar per kg,
we probably are in the right ballpark. So, around 150 000 Dollar
per launch.

This is _very_ low compared to all the other costs. A launch
cost around 50 to 60 million dollars now, if I remember the
figures right.

That’s just the problem: you’re only accounting for the cost of the fuel, whereas I would consider large parts of “all the other costs” as inherently part of the efficiency equation. After all, if we found a way to eliminate the boosters entirely, the savings is obviously not just from the decrease in fuel.

--
"Also . . . I can kill you with my brain."
River Tam, Trash, Firefly