Microeconomics (adapted to effort and belief):
My willingness to trade one understanding or belief for another is measured by a marginal rate of substitution (MRS): the maximum amount of effort in one belief system I will sacrifice to obtain a new understanding or belief. The marginal rate of substitution refers to the trade-off (rate of substitution) of my prior understanding for a marginal (small additional or incremental) change in my new understanding. My marginal rate of substitution of say Christianity for some other form of spiritualism is MRS = ∆B/∆Z, where ∆Z is the amount of effort spent being in, say Christianity, I will give up to get ∆B, this other form of spiritualism, or vice versa, and Christianity (Z) is on the horizontal axis. The marginal rate of substitution is the slope of the indifference curve.
Note that asymmetric nature of this curve.

And some other indifference curves that don't really exist in humans, but to illustrate a point:


