According to Bankrate.com, Money market funds are currently yielding 1.36%. 252 years of 1.36% yields a factor of 30.087, which would turn your $800 into $24,069.86.
Now, you may say to yourself that interest rates probably wouldn't stay at 1.36%, but would instead probably average somewhere around 4%. 1.04^252 = 19,606.56, which would turn your $800 into $15.685 million.
Of course, if you went 252 years in the future, you couldn't just get dollars and bring them back now, as they'd probably be different looking by then. Instead, you would have to go to an antique store, if they existed, and buy old dollar bills from the present. How much would old coins and dollars cost? Probably 20:1, so you'd have less than a million after all that, and you'd also have to assume no escheating law, and no taxes. But Social Security will be broke by then unless they raise taxes a lot.