  In their seminal paper, Karp, Vazirani and Vazirani (STOC'90) introduce the
online bipartite matching problem, and the RANKING algorithm, which admits a
tight $1-\frac{1}{e}$ competitive ratio. Since its publication, the problem has
received considerable attention, including a sequence of simplified proofs. In
this paper we present a new proof that gives an economic interpretation of the
RANKING algorithm -- further simplifying the proof and avoiding arguments such
as duality. The new proof gives a new perspective on previous proofs.
