It was minted in phildelphia, those coins have no mint mark, how rare? there were 6,851,765,000 minted.
Worth about 1 cent in my book; in Sm condition, which yours does not appear to be, I'd be a buyer at $0.05.
You should pick up a copy of the Red Book- It gives a lot of information including mintmarks, which coins have them and which dont, years they started/ended, etc. The only 1990 cent worth having, IMO, would be a Proof 1990 No S cent, of which this coin is not.