Good question actually.

Let's do some heavily simplified calculations:

The range R is directly proportional to: ln(sqrt(M0)-sqrt(M1))

So let's set x as our payload variable such that: x = 0 when Mfuel = MaxFuel and M0 = MTOW

M1 will then be M when the all the fuel has been spent. This means that when x < 0 we have max fuel, and when x > 0 we start trading fuel for payload.

We can then write: R \~ ln(sqrt(min(MTOW+x, MTOW) - sqrt(max(MTOW-MaxFuel, MTOW-MaxFuel+x)))

Let's now consider payload in the simplest way possible, let's say when using max fuel and x = 0, the aircraft should be at MTOW, that is: P = MTOW-OEW-MaxFuel+x

And let's now say that the revenue obtained from a flight is a product of range and payload, that is: REV \~ R*P

Taking the rough values for an A321 XLR in metric tons: MTOW = 101 MaxFuel = 33 OEW = 50

We have: REV \~ (18+x)*ln(sqrt(min(101, 101+x)) - sqrt(max(68, 68+x)))

When you put this into Wolfram Alpha and plot it, you get maximum revenue at, you probably guessed it: x = 0 :)

This means, for this very simplified calculation, that you'd get the most money when filling the tanks to their maximum capacity and then adding payload until you reach MTOW.

This is really obvious, if you think about it, as there's a discontinuity in the range vs. payload graph once the tanks are full, and you start getting a much lower increase in range for every decrease in payload.

Reality will probably not be exactly like this super simplified scenario, but it probably isn't far off either.