What is 1928 mercury dime worth?

Depending on condition, the dime could be worth anywhere from just its silver content, (about $1.50 at the time of writing) to a few bucks if it is uncirculated. It isn't a particularly rare date, so it is worth silver content if it isn't in great shape. Just remember, cleaning old coins harms the value of them and degrades rather than improves the condition.