Realized today that the more precise form of "money can't buy happiness" is actually something like "money's purchasing power with regards to happiness decreases marginally as spend increases".
Through that lens, the optimal amount of money to acquire is equal to the point where the marginal unhappiness gained acquiring money equals the marginal happiness gained from spending that money.
In this optimization problem "spending money to acquire happiness" doesn't necessarily equate to literal spending (could also be saving, investing, donating, anything one could do with money that might directly or indirectly bring more happiness that unhappiness).
Of course, the trouble with this kind of optimization problem is that its hard to quantify and belabored by tons of second and third and fourth order terms and all that.
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