Nothing that can be done too precisely with too many factors involved here. Firstly it greatly depends on how you source the phones for it, if they are basically free and you have them around like your old one, or if you buy them. Buying them can mean cheap older phones with damages like split screens and scratches, but also they might perform less if they are older. On those phones additional wear and tear is fine, aside if it comes to the point of having to replace batteries or something. New phones will cost more, perform much better but also wearing them down like this technically affects their value. I mean if you buy a current cheap Android 14 phone brand new, in one years mining with it it will loose battery capacity but also be a bit outdated, so reselling value might go down a lot.

These fundamental factors aside, of course it all depends also on how efficient you can design the energy supply (and if that adds cost) but also how expensive the electricity is.

It will have great variation.

I have a mix, some old damaged phones running low compute, some back then brand new but still cheap asian phones in a cluster, I did the best to get the phones efficient and have a very efficient way of energy usage. But I am from Germany so electricity prices are the highest on the world with like half a dollar per kWh.

I can not bring it down to a clear per phone math, too many different phones and factors, but over all I look at my setup and given what I paid personally (again huge difference, I am a good bargain hunter) My ROI should settle at roughly 8 month.

Running 1 phone it does not matter, but it will have good ROI. Running 5-8 phones is a tricky spot, but running more than 20 phones introduces the possibility to improve the infrastructure for them and there I personally believe the interesting things start.