In The House on Mango Street, why do the girls want five dollars? What are they willing to give Esperanza in exchange for the money? Why does Esperanza feel that five dollars is cheap?

In Sandra Cisenros's novel, the protagonist Esperanza has become friendly with her neighbor Cathy. Rachel and Lucy live across the street from Esperanza, and they come to her one day while she is with Cathy and present a scheme to buy a bicycle. Rachel asks for five dollars to buy Tito's bicycle, offering eternal friendship in return.
Cathy is skeptical, but Esperanza is easily swayed and agrees to give them the money. Although Esperanza suspects that this new arrangement may result in the loss of Cathy's friendship, she rationalizes it by thinking of the new friends she has gained along with the partial ownership of a bicycle, which seems like a good return on the money. Once they do succeed in buying the bike, the three of them take turns riding it around their neighborhood.