What is the value of a 100 dollar bill printed in 1990 that has been misprinted with a reverse image of the back printed askew over the front?

This sounds like an offset error and is quite valuable - maybe in the range of $200+. You need to get appraisals from a couple of dealers who specialize in error currency.

An offset error happens when a sheet of paper gets stuck and doesn't go through the printing press. Ink is transferred to the opposite side of the press instead of the paper. When the next sheet gets unstuck and goes through, it picks up not just the ink it's supposed to have, but the reversed image from the opposite side of the press.