befnme;
I actually tried this a long time ago to get battery power across my yard with just one wire.
It had plenty of voltage from ground to + wire from the battery but as soon as I added any load it dropped to nothing. Basically once you draw any current [amps] the earth resistance eats the power.
Thats why it "works" for fencers to use the earth as that application uses high voltage pulses that do not draw any current.
Just a real world experience in case you are not convinced it is not going to work.
You can do the rods in the ground test someone mentioned to see for yourself.
We have all struggled with the cabling costs and installation hassles and the true bottom line is "you need copper wire".
One trick many do not realize you can use is that you can parallel multiple cables to increase the apparent size.
As in you install the #10 you can afford today and this works but is lossy. Later you can install another cable run and parallel it with the first to increase the apparent cable size.
Good luck with it.
TomW