I did MATHS to the LotFP classes

I did MATHS to the LotFP classes, to work out what goes towards their different XP costs to level up, with the intention of putting together a set of guidelines towards building your own classes. Strongly inspired by the 'Building the Perfect Class' pdf over at Breeyark, which I stole the idea from (although the numbers are different). Go check it out, it's fascinating.
I have no idea if these numbers were actually what resulted in Mr Raggi picking the numbers he did. I suspect things got tweaked up and down based on playtesting, since some abilities synergise niceley together to be worth more as a package than in isolation, etc etc.
Anyway, here's the results.

* * *

Assembling your class
You basically ‘pay for’ abilities in XP costs to level up. Add them all up, and that is how much it costs you to go from 1st to 2nd level. Then double the cost for each level thereafter, until 9th level. After 9th, it costs as much to go from 9th to 10th (and from 10th to 11th and so on) as it did to go from 8th to 9th.

You start out needing 1000 XP just for existing. You must pick a hit-dice and save progression, obviously, but the rest is optional. If you're a spellcaster, you must pick both a casting type and spell list.
You are strongly discouraged not to take all the best abilities (particularly those that just give a 1-time benefit at level 1) and just resign yourself to never levelling up; if you do this I hereby declare that you must buy the GM a drink each session you play this character. I wrote it down, so now it's the law.

For your hit dice, pick one of the following:
-100 if your HD is a d4 (or +1 from 10th level)
+0 if your HD is a d6 (or +2 from 10th level)
+100 if your HD is a d8 (or +3 from 10th level)
+200 if your HD is a d10 (or +3 from 10th level)
if you go mental and include d12s as hit dice, that would be +300.

For your saves, pick one of the following:
+200 if you save like a cleric
+0 if you save like a fighter
+0 if you save like a magic user
+0 if you save like a specialist
+200 if you save like a dwarf
+100 if you save like an elf
+300 if you save like a halfling
This was the least well-researched bit, as I don't have the sort of insane analytical skills to line all the different save progressions up against one another and exactly how they compare. Leave a comment if you think I made a mistake here.

For skills, pick any of the following you want:
+200 for each skill point gained when you level up - you may take this multiple times and gain 1 extra skill point per level each time you do.
+50 for each skill point gained at first level - you may take this multiple times and gain 1 extra skill point at first level each time you do
+100 for each skill that starts at 2-in-6, increasing at the same rate an Elf’s Search skill does – you may take this multiple times, and it applies to another skill each time you do.
+200 for each skill that starts at 3-in-6, increasing at the same rate a halfling’s bushcraft or dwarf’s architecture skill does – you may take this multiple times, and it applies to another skill each time you do.
+300 for each skill that starts at 5-in-6, and doesn’t improve as you gain levels – you may take this multiple times, and it applies to another skill each time you do.
You could conceivably add new skills (such as swimming, medicine, 6th sense, disguise or what have you) that use the same x-in-6 mechanic.

If you are a spellcaster, you must pick your casting method (prayer or spellbooks) and spell-list (chaotic or lawful), and can pick any other stuff from the big list. If you aren't a spellcaster, you can't pick anything from here at all.
Pick one of these two:
+500 if you cast like a cleric, by praying and receiving your allotted spells from a set list. Use the Cleric's Spells per Day progression.
+800 if you cast spells like a magic user, by using a spellbook that you can copy spells into, that starts out with only Read Magic in it. Use the MU or Elf's Spells per Day progression (so far as I can tell, they're identical).
And one of these two:
+0 if you cast from the cleric’s spell list (which also makes you lawful)
+300 if you cast from the magic user’s spell list (which also makes you chaotic)
(Option: you might want to alter these spell lists to give custom spell lists for, say, illusionists, cultists, etc. You'll definitely need to add spells above 7th level if a character casts lawful spells from a spellbook.)
And then any of the following you want:
+50 if you can create spell scrolls
+50 if you can create protection scrolls (option: technically nothing about this requires spellcasting, so maybe you can have it even if you don't have spells).
+50 if you can create holy (or unholy) water
+50 if you can create potions
+50 if you can create wands and staves
+50 per extra spell in your spellbook when you start the game (for spellbook users)
+100 if you can cast spells while heavily encumbered and/or with a hand full.
-100 if you require a holy symbol (or equivalent) in order to cast.
-50 if you cannot move at all if you wish to cast.
I, for one, have always felt that clerics get it too easy what with getting full access to their whole spell-list and needing less XP than magicians, but hey, this is how the numbers for LotFP shook out. I guess the potential to learn fucking any spell

Combat training (take any you want):
+100 to be able to use the Press and Defensive Fighting options.
+100 to get an additional +2 AC when parrying.
+100 to get an additional +2 to hit when aiming (no class gets this, but I added it for symmetry).
+300 to get +1 to hit per level, like a fighter. (This is on top of the +1 to hit all level 1 characters get).
+100 to reload guns faster, like a fighter.
+100 to have maximum HP at first level, like a fighter.
+200 tax if you’re a Fighter, the Best Class, and also I couldn’t make the numbers add up without including the Fighter Tax.

Other abilities:
+100 to get +1 to an attribute modifier -  you can take this multiple times, each time you do it applies to a different attribute.
+100 to get +1 AC so long as you aren’t surprised.
+100 to apply your Constitution modifier after 9th level.
+100 to ignore the first five items towards encumbrance, like a dwarf
+200 to be innately chaotic/lawful and thus immune to various things that affect mundane beings, like an Elf is (obviously, this makes you chaotic/lawful).
+50 to be immortal and never age
+100 to be surprised 1-in-6 less often
-50 to be unnatural and vulnerable to holy water and so on (obviously, this makes you chaotic, or perhaps lawful).
-100 to be small, and so unable to use large weapons and forced to use medium weapons two-handed.
+variable for abilities you made up (discuss this with your GM, obviously), using the amounts for similar abilities as a guide.