Wow what an interesting question! I will try to frame my answer in the perspective of finance and economics.
I think a market rate for ang bao is not very feasible from the lens of economics. Prices of any product or service is both affected by demand and supply forces, based on how society values it. The Invisible Hand theory of economics, would state that the price of a product or service will fall into equilibrium, where both customers and suppliers of a product or service can agree on a stated price. The problem with setting market rates arbitrarily is that either demand or supply will exceed one another, and the market will be in disequilibrium.
This is precisely the scenario you laid out above, where the amount of money in the ang pow seems that it should be appropriated according to how wealthy the person giving out the ang pow is. Markets in disequilibrium aren’t a good sign, as external parties like governments need to step in and intervene. Thus, economists would say that the everyone is happiest when you simply let the market run on its own without setting market rates.
Hence, everyone will probably be happier if they can just decide how much ang pow they want to give!
At the end of the day , it's a famiy / personal choice to give more or less. Maybe take it easy and rather remember the original meaning and purpose of it so that its chill during CNY holday ? :)
Origin of Ang Pow:
"In China, during the Qin Dynasty(https://en.wikipedia.org/wiki/Qin_Dynasty "Qin Dynasty"), the elderly would thread coins with a red string. The money was referred to as "money warding off evil spirits" (Chinese: 壓祟錢; pinyin: yāsuì qián) and was believed to protect the person of younger generation from sickness and death.[citation needed(https://en.wikipedia.org/wiki/Wikipedia:Citatio... "Wikipedia:Citation needed")] The yasui qian was replaced by red envelopes when printing presses became more common and is now found written using the homophone for suì that means "old age" instead of "evil spirits" thus, "money warding off old age" (Chinese: 壓歲錢; pinyin: yāsuì qián). Red envelopes continue to be referred to by such names today. "