I agree.
Alternatively, instead of considering the monthly average, I like to frame it as how much I would be willing to pay to get X done. For example, finishing SICP is probably the most significant intellectual accomplishment in my life. Would I pay $1000 for a magic pill that implants the experience and the skills I gained from SICP into my mind? Absolutely. Considering the value it brings in terms of future earnings and pure joy, it might even be worth more than that.
And that’s just one goal that I picked out. I could make similar arguments for a few other ones. Just think of the implicit value of the health benefits I gain by practicing yoga or meditation.