Tips shouldn't be compulsory imo, because if you make tips compulsory players will stop accepting services from other players. It costs about $250 for medic/firefighter services which is much cheaper as compared to healing from a hospital (around $900 if you are on less than 25% health). That is the only reason why players are willing to wait for medics to arrive.
And yeah tip amount could be increased.... but again players will never tip 15k. (I personally never give more than $690)