Each exchange has been completed for "free." That means that no money was exchanged, at least for right now. However, a rather complex set of connections and expectations have grown up between the artists as the project progresses.
If the exchange drawings as a whole were to sell at some point in the future, the proceeds would be divided among the artists, probably based on the number of drawings completed plus model fees for sitting for a portrait, minus some basic costs like paper. That model would necessarily mean that Walsh would receive more compensation since he will have completed exactly half of the drawings. Another model would simply be to divide the proceeds by the number of artists. If the drawings do sell, the compensation model would be determined by discussion between all the participating artists.
Currently, as organizer of the exchange, Walsh is in temporary possession of all the drawings, acting as a kind of steward for the project. This allows Walsh to scan drawings for the blog and potentially organize their exhibition. At some point, if the drawings do not sell as a group, the exchanges will be completed, and Walsh will give each artist their portrait, while retaining the set of those drawn of him.
Central Park Portrait Exchange is organized by artist Peter Walsh.