Thanks for the link Dragonblade!
Okay, the D&D XP for monsters could be something like:
1st: 100 XP
2nd: 125
3rd: 150
4th: 175
5th: 200
6th: 250
7th: 300
8th: 350
9th: 400
10th: 500
11th: 600
12th: 700
13th: 800
14th: 1000
15th: 1200
16th: 1400
17th: 1600
18th: 2000
19th: 2400
20th: 2800
21st: 3200
22nd: 4000
23rd: 4800
24th: 5600
25th: 6400
26th: 8000
27th: 9600
28th: 11,200
29th: 12,800
30th: 16,000
Minion 30th: 4000
Elite 30th: 32,000
Solo 30th: 80,000
Of course this doesn't explain why the Pit Fiend is 18,000 XP rather than 16,000, but it could be that monsters can use everything from their level, up to, but not including the next. So the 26th-level range is 8000-9599 XP.
A few musings. If 4 minions = 1 standard monster, 2 standard = 1 elite and 5 standard = 1 solo, then the following seems logical.
Standard = Minion +8
Elite = Minion +12, or Standard +4
Solo = Minion +17, Elite +5 or Standard +9
Ergo (while the math itself may need to be rejigged?) a 26th-level Elite Pit Fiend would be a potential:
21st Level Solo Monster
30th-level Standard Monster
(Hypothetical*) 38th-level Minion Monster
*
