An exchange in which all the benefit is on one side. This proverbial expression is founded on an incident related by Homer in the Iliad. Glaucus recognises Diomed on the battle-field, and the friends change armour.
For Diomed's brass arms, of mean device, For which nine oxen paid (a vulgar price), He [Glaucus] gave his own, of gold divinely wrought, An hundred beeves the shining purchase bought.
Pope Iliad vi.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894