Actually it doesn't have to involve digging deeper at all. When a businessman considers if he should build a mine, he goes through a list of expenses. How much will it cost for a rail line, how much for machinery, how much for the buildings, how much for the electric, how much for the enviromental impact studies, how much for attracting the workers, how much donated to local towns for goodwill, how much will they tax me, how much will the state tax me, how much will the fed tax me.
Then he compares that to how much ore is on site, how many years it will take to extract, and how much he can sell that for. Total revenue - total expenses > 0? Economical to extract.
There are countless deposits considered 'uneconomical' not because they're half a mile below the surface but simply because there's not enough there to pay off the initial investment of machinery and infrastructure at the current prices and tax rates. Fields of iron ore, thousands of tons each, sit untouched in Minnesota simply because it takes millions of tons to pay off the investment.
Curiously I've read something about the moon being an excellent source of hydrogen isotopes for fusion. On the sun my ass!