If there is no mint mark for an 1889 half dollar how much is it worth?

The lack of a mint mark means your coin was minted in Philadelphia. Retail prices range from $270 if worn, up to $350 if only some wear shows, to $400+ if only slightly worn.

If you plan to sell, get a couple of competing appraisals. Remember that a dealer will pay wholesale, about 30-40% less.