I try and keep it simple to keep the campaign moving. It is just assumed that a mentor will have more experience than the student. Some students can best their mentors, though, understanding aspects of training in a different way. Here goes:
Classes, Feats, or Skills, Trained or otherwise, possessed by party members may be trained ahead of time at no cost. Only Trained Skills, Feats, or Classes that party members do not possess require a mentor. The first rank in a Trained Skill requires 5 days of training with an NPC mentor for 50gp. Extra ranks in trained skills come naturally and represent one’s focus in that skill, and thus take no time or money. Some Feats can not be trained ahead of time at the discretion of the DM. Feats that must be trained and are not possessed by a party member require 10 days of training with a mentor for 100gp. A new class not possessed by a party member takes 20 days to train at a cost of 200gp per level of the mentor.
Classes, Feats, or Skills, Trained or otherwise, possessed by party members may be trained ahead of time at no cost. Only Trained Skills, Feats, or Classes that party members do not possess require a mentor. The first rank in a Trained Skill requires 5 days of training with an NPC mentor for 50gp. Extra ranks in trained skills come naturally and represent one’s focus in that skill, and thus take no time or money. Some Feats can not be trained ahead of time at the discretion of the DM. Feats that must be trained and are not possessed by a party member require 10 days of training with a mentor for 100gp. A new class not possessed by a party member takes 20 days to train at a cost of 200gp per level of the mentor.