How much is a 1950's series c 10 dollar bill with an upside down flag on it worth?

A 1950's 10 dollar bill is around $20-25.

The urban legend of an "upside down" flag on the back of the bill has been debunked many times by the Bureau of Engraving and Printing, the agency that prints US currency. The plates used for printing older $10 bills were engraved deeply and while the flag appears to be upside down, it's actually just an artifact of the way the ink flowed.

That's why the flag was never "corrected" even though that style of bill was printed for decades - it was never wrong in the first place.