Your Milks
I slapped this together. If there’s any questionable information let me know.
I made some edits. I’m not sure if there are regulations about what can be called milk but some of Costco’s products are called “beverages,” not milk. In any case, a manufacturer can choose to add more or less water, flavoring, sweetener, etc. so ymmv (your milks may vary) when it comes to costs and nutritional value. Hopefully the comparison among strictly Costco products is still useful.
Cow’s milk is bad for the planet due to the quantities of carbon emitted into the atmosphere to produce it, the amounts of water and land required, and because cleaning up after dairy cows ends up contaminating water sources.
Coconut milk has been alleged to be produced using captive monkey labor [1]. It’s the most expensive commonly available option. It has tons of fat. (This is true if it’s undiluted, not necessarily for coconut “beverage”.)
Soy milk is problematic due to the sheer volume and location of soybean production. It accounts for a significant amount of deforestation (demand for beef is the biggest factor and this in turn also drives soy demand) [2]. It contains compounds similar to human hormones, though this issue may be exaggerated [3].
Almond milk adversely affects bees because so many of them are required for almond tree pollination. They are dying in large numbers for a variety of reasons including pesticides, disease, and disruption of their hibernation cycle [4]. Almond milk also requires large amounts of water, though significantly less than dairy milk. It has less protein than milk from dairy. Of alternatives to dairy, it’s currently the least expensive. It costs about 50% more than dairy milk.
Oat milk seems to be relatively benign but it contains much less protein than milk from dairy. It’s expensive, but you can make your own [5]. I guesstimate about a 2 cup yield of unflavored milk from 1/2 cup (~40 g) of bulk rolled oats ($0.054 at SLC WinCo). The end result is oat milk at about 1/5 the price of dairy. The oats can also be re-used (to make cookies or whatnot).
The table summarizes cost (Costco prices in SLC, June '21) and selected nutrition information for various milk sources. Updated table.
The graph below compares the impact of milk products [6]. Emissions are kg of CO2.
[1]Costco dropping Chaokoh coconut milk over monkey claims, PETA says
[2]Soybean - Wikipedia
[3]What Does Soy Actually Do To Your Hormones? | Discover Magazine
[4]'Like sending bees to war': the deadly truth behind your almond milk obsession | Bees | The Guardian
[5]How to Make Oat Milk Recipe - Love and Lemons
[6]Climate change: Which vegan milk is best? - BBC News