[AtlasBarfed]

300,000 gallons, .01/6.4 gals

so .01 * 300,000/6.4 is... $500.

How often do they need the water?

[myfavoritetings]

That's 500 for one acre so multiply by 300-400 for a typical farm and it cuts into already fairly small margins on farming

[sandworm101]

Corn will need about 24" of water each crop. So, if there is no or little natural rain, two acre-feet of water per acre. There is no concievable method of transporting such amounts inland. It would require energy enough to reverse the flow of rivers.