High Level Experience requirements
This information is based on relatively reliable information, and is moderately accurate.

From Lok's Experience formula:  X(n+1) = X(n) + r*g*176xp = X(n) + Delta

X(n+1) is the experience to level.
X(n) is last levels experience to level.
r is the racial penalty nomalized to human.
g is the guild penalty normalized to artisan.

I have found this formula to give slightly HIGHER values that those in the game. So, by the time these small errors add up, you might end up saving 5 million XP by the time you reach higher levels.

Now Lok calculated that g = GF/8, giving:
Warrior, Explorer, Barbarian
Ninja, Villain
Magi, Sorceror
Vamp figured out racial penalties a long time ago:
Giant, Gnome, Ogre, Troll(6)
So with these figures, and using a simple sum to n terms for an arithmetic progression, i.e. Sn = n/2[2a + (n-1)d], we come up with the formula:

XP(lvl) = r*g*lvl/2*[354 + (lvl-1)*177]

But this does not take into account penalties, which would have to be multiplied to this total.  So, it is therefore possible to figure out how much experience it takes to get to lvl 999 in any guild. If you are playing a serious multiguild character, and the assumption that it is now virtually impossible to level out of penalties is correct, then your penalty and whatever guilds you are running will get to 1.5 and stay there.

Also, since seems to take the same amount of experience to get to two specific levels in different guild regardless of how you level them, we can apply 1.5 as a base multiplier to XP in above formula.

Now Finger corrected me on one point below: 1.5 is actually a extra 150% exp required - so we will actually need to multiply by 2.5!!  So:

Working with getting lvl 999 in artisan with 1.5 penalty:
XP(999) = 999/2*(354 + 998*177)*2.5 = 221028750 XP
I will refer to this a Base Total.(BT)

From this we can figure out XP required for any other race/guild combo just by multiplying by r and g.
e.g. Human ninj/sorc requires:
BT*2.625 + BT*3.375 = 1,326,172,500 (1326M)
This is actually better expressed as BT*(2.625+3.375)

OR a dwarven warrior/cleric/thief (as favoured by many) needs:
BT*1.1055*(2+4+1.875) = 1,924,234,855 (Yes: 1924M!!)

So XP required = BT*r*(sum[g])

Comparing a lvl 999 ninja to a lvl 500/500 ninja/sorc:
999 ninja = 232,080,187 XP (232M)
500ninja/500sorc = 332,538,750 XP (333M)

So, the pure ninja requires less experience, but can you imagine finally switching to another guild???
Hmmm... a rampaging mage on lvl 1 with a bronze dagger and 11 or so swings.
Also from that example, it takes a quarter of the XP to get to lvl 500 in each as opposed to lvl 999, and at higher levels your skills don't increase comparatively as much as they do at lower levels. So, it maybe best to just stop at lvl 500 instead of going on to lvl 999...
Unless you need to for item use. :)

FINAL note to put things in perspective: the approximate number of slave kills it will take to reach these values.(Slave gives around 35 XP) And yes, this is sadistic.
lvl 999 ninja/sorc - 37.9 Million
lvl 999 warr/thief/cler - 55 Million
lvl 999 ninja - 6.6 Million
lvl 500 ninja/sorc - 9.5 Million
So start killing those slaves.  *grin*