How much does a Lyft ride cost from Mission street near 18th street to California street near Polk?

A Lyft rider once went lyfting with two other friends from 18th and Mission St. to California and Polk St. He showed me his screen, which the Lyft app suggested an $18 donation. According to this Lyft passenger, a taxi would charge about $20. The rider was saving $2.00 + a tip to take the Lyft.