Without going on with the almost certainly absurd claim you can be perfectly healthy, indefinitely, eating one shake.. There does seem to be a lot of interest, I think mostly from people interested in the saving of time and still getting good nutrition.
To those people, I highly recommend green protein smoothies.
They aren't cheap, really, but they are fast and done right they taste good and will take an otherwise mediocre diet to better than 99% of people without much other change (I am not a doctor, just an observer).
Some fruit, some leafy greens, some nuts, optionally some protein powder (my favorite had been pea protein, but my nutritionist recommended alternating protein sources every few weeks of doing it regularly).
Adding in some thing like vitamineral green and you are really doing alright. Spending about 3-400 USD on a good blender and you can have an über healthy, delicious meal replacement in minutes with about 30 seconds of clean up.
A mediocre blender still make the experience less enjoyable when using nuts and tough greens like kale.
Expect an 800 calorie shake to cost maybe 3-5 bucks to put together depending on your tastes, whether you go organic, and access to produce.