How much is a 1989 penny with no letter under the date worth?

One cent.

It's worth one penny. About 7.26 billion were minted that year.

The fact that there's no letter under the date means it was minted at Philadelphia. Even though all other Philadelphia coins started using the P mint mark in 1979 or 1980, the cent was excepted because occasional "short runs" of cents are made at West Point, and the Mint didn't want to create instant rarities.