EDIT: My math was totally wrong. 10 trillion dollars, 0.1x world GDP. (Still a lot of money though) Leaving the old text below so the replies make sense.

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To move 6,000,000 megatons of fuel into low earth orbit would take 111,111 Falcon Heavy launches, at a total cost of $10,000,000,000,000 dollars. (10 quadrillion dollars, 93x world GDP)

Something that takes the total global economic output of the planet a century just to lift the fuel into orbit doesn't read as plausible in the next 50 years to me.