The topic at hand is leveling up in AD&D 1e and the fees, mentors, etc. The DMG tells us;
"Experience points are merely an indicator of the character's progress toward greater proficiency in his or her chosen profession..."Interesting, isn't it? Gary goes on to outline a system of rating a character and the roleplaying involved and then discusses a program where characters will be out of play for weeks and spend money, sometimes vast sums, to level up. The fun thing? Going from 2nd to 3rd level should typically cost you so much money that you technically should be 4th level already.
"The gaining of sufficient experience points necessary [for] a character to be eligible to gain a level of experience but the actual award is a matter for you, the DM, to decide"