One of my favorite authors for Dragon Magazine was Paul Crabaugh. For the handful of articles he wrote for D&D, I always appreciated his Class and Monster Generators. One the stated goals for Andras was to have a similar class creation system for DMs to use to better customize their world.
In 2e, a system was presented, but it states very plainly in the beginning that existing classes cannot be re-created using this system. In fairness, the same was true for Crabuagh's Customized Classes In Dragon #8. In some ways, it never felt right that I couldn't just re-create existing classes.
After a few months of experimenting with various systems, the conclusion is that the xp numbers are largely arbitrary. That may not be a surprise to anyone, but I thought that there might be a way to get reasonable close to the numbers, but I guess it wasn't meant to be.
Oddly enough, I can reach the numbers exactly for Basic Fantasy. Just not anything similar to 2e.
In the various experiments, there was a tension between the value of combat prowess versus the value of magical access. If combat was undervalued, the warrior group gained levels rapidly and the rogue group tended to have too many skills for the same xp levels. If magic was undervalued, there was little incentive to be a magic-user.
So as I wind down to a final version of the class generator, I find that Priests and Psionicists have the same XP progression level. Personally, I like that because I tend to characterize Psionic characters as anti-clerics. In other words, in function, they appear to generate magic in the same way, the difference is the source. Priests gain power from a deity and a psionic character gains power from within. It serves to make the psionic characters a persecuted group and also allows for deities to grant more anti-psionic miracles.
However, when the specific mythoi options for priests are used, their progression is a bit faster in my customized class system than a standard cleric. The main reason is that a specific mythos priest has access to half of the miracles that a cleric can access. In 2e parlance, that 8 schools with major access and 1 school with minor access versus 3-4 schools with major access and 1-3 schools with minor access.
The effect is not as pronounced with mages versus specialty wizards because of the bonuses that specialists gain.
In case you are wondering, my system makes the standard 2e paladin need almost 3000 xp to reach second level. Needless to say, I'll probably end up re-doing the paladin anyway. I want a battle priest/paladin/avenger class where alignment isn't so much a restriction as the specific deity served.
Maybe most surprising of all may be the fact that psionic powers are the most expensive form of magic.
I hope to have the generator ready in spreadsheet, pdf and php format soon. If anyone is interested, I will be happy to share the current working spreadsheet. I would upload it to the site, but it changes daily as I crunch numbers.