Jamieson, Fausset and Brown on Hosea 3:2

2. I bought her--The price paid is too small to be a probable dowry wherewith to buy a wife from her parents; but it is just half the price of a female slave, in money, the rest of the price being made up in grain (Ex 21:32). Hosea pays this for the redemption of his wife, who has become the slave of her paramour. The price being half grain was because the latter was the allowance of food for the slave, and of the coarsest kind, not wheat, but barley. Israel, as committing sin, was the slave of sin (Joh 8:34; Ro 6:16-20; 2Pe 2:19). The low price expresses Israel's worthlessness.