Albert Barnes Commentary 1 Samuel 6:4

"Then said they, What shall be the trespass-offering which we shall return to him? And they said, Five golden tumors, and five golden mice, [according to] the number of the lords of the Philistines; for one plague was on you all, and on your lords." — 1 Samuel 6:4 (ASV)

It was a prevalent custom in pagan antiquity to make offerings to the gods that expressed the particular mercy received. For example, those saved from a shipwreck offered pictures of the shipwreck, and this custom still exists among Christians in certain countries.

The plague of the mice is analogous to the plague of frogs in Egypt. The destructive power of field mice was very great.