What is the value of a 1987 US dime with no mint mark?

PCGS doesn't show any reports of 1987 dimes struck without mint marks. While that error has been authenticated on some 1982 dimes all 1987 circulation dimes should have either a P or D above the date, and proofs should have an S.

Because dimes are small, it's common for details near the edge to be blurred or indistinct. If you don't see any evidence of a mint mark even under a magnifier, you should have the coin inspected in person by someone who specializes in error coins.