Plus if time is an issue, the player could always use a faster, more expensive method; if cash is an issue, they could use a cheaper but slower method. I'm not sure what exact methods gemeos uses for his calculator, but there are plenty of skills where this trade-off can be effectively made. For example, in terms of saving cash, doing extreme defence pots rather than overloads for, say, 150m herb xp would save 1.75B while still giving a decent xp rate.