Per feather, as
@CrusaderMichael proposed, might be a good guide base but there is no black and white solution. I nearly always go for the chest with the most feathers because I need to keep moving through the grand prizes to make it to the end. The difference in cost of chest vs feathers vs contents is not in proportion, so
spending less by going with cheaper chests may stretch the currency farther, but it is very high risk of not making it to the end because its fewer feathers don't add up. If all chests have the same feathers, then the cheapest, of course. But I also look at the content percentages. If the highest and second highest chests have nearly the same chance of daily prize acquisition, I also look at the consolation prizes because most likely that's what I'll get, so choosing the most attractive chest overall. It is in cases like this where I'll do four feathers instead of five. Also, just because one chest has a higher chance of daily doesn't mean the others won't. Sometimes, on a whim or whatever, I'll choose a lesser chest that doesn't make sense and get the daily there! So... strategy + intuition + luck.
[I made it through, getting a second base, with a day to spare. Three lava eggs, three burning eggs, three riders, three temple/embers, two pavilions, and three saddle forges (saved in inventory for emergency disenchantment).... So, I did pretty good without stash outpost or purchasing currency. My point here isn't to boast my
luck, but to just demonstrate what is possible. There are many different approaches that can be successful; I'm just sharing my strategy in case it is helpful to anyone.]