There might be an absolute or relative requirement.
20k ist almost nothing. A stamina 100 girl can train this naked in a day. Coaching lifts the coached girl up to the coaching girl. You can easily train any girl from 0 to more than what you would get from coaching. You do know, coaching costs 14 days, do you? It gets less if the coachee has more stats already, but as far as I can tell, minimum coaching time is 3 days.