Because you asked... based on its particular version of Horstmann Co. backmark ("HORSTMANN * PHILADa"), your button dates from 1893 to no later than 1902 (when production of that button-emblem ended). So yes, it could have come from your great-grandfathers Spanish-American war (1898) uniform.
Edit: I forgot to answer your question, "Any idea what these [buttons] are worth?" It is unwise to rely on Ebay for dollar-values of relics. Anybody can buy these chicken-head eagle buttons for $2-to-$3 at various Militaria shows and gun-shows. That's because they are far more common than (let's say for example) your Kentucky State Seal button from the same time-period. There were literally millions of Enlisted-men (privates, corporals, sergeants) in the US army between the 1870s and 1902.