Fuel is not cheap. Fuel is heavy and payload reduction is money lost.
_Lifting_ that fuel is NOT CHEAP. See also, the rocket equation.
But we're essentially arguing whether max payload per launch or payload efficiency to orbit is more important.
So yes, fuel is cheap.
With one and done you have a larger mass fraction to work with. Reusable spacecraft operate on much tighter constraints. Every single wasted pound in Space X’s design likely costs them 100’s of thousands of dollars in lost profit over time.
Reusable means cost savings, but it has a dramatic reduction in cargo capacity. This means more trips to get the same mass into orbit, which means mire fuel directly used and every extra lb in the design is then carried up multiple times while further requiring more trips.
Or, equivalently, in one trip with a bigger rocket you have left over from another launch, because you didn't ditch it.
Which again demonstrates reusability is really expensive, though clearly not always as expensive as a new rocket.
Remember while the first stage is significantly more valuable, it's not just a question of fuel the upper stage is destroyed with both approaches. So, combined with significantly lower payloads the cost per kg does not drop as much as you might think.
It doesn't make fuel cheap, let alone very cheap.
Actually another significant part of SpaceX business is modern engineering and modern mean of production. They used to launch without recoverability for some high orbit payloads and they were competitively priced for that too.