Nonsensically, an NPC's gear value can be based on its ECL
While Epiphanis's answer points to the Dungeon Master's Guide wherein it says that
Many monsters advance by adding class levels (see the Monster Manual). To determine treasure for monsters with class levels, first give them equipment. Use Table 4–23: NPC Gear Value (page 127) and use just their class levels to determine the value of their equipment. Then generate their treasure according to their monster entry and the rules under Building a Treasure, below. This may generate more items that the monster can use, and that’s fine.… (51)
This method means the monster has adventuring gear—its NPC gear value—in addition to its normal treasure.
But the Monster Manual says that
If you choose to equip a monster with gear, use its ECL as its character level for purposes of determining how much equipment it can purchase. Generally, only monsters with an Advancement entry of "By character class" receive NPC gear; other creatures adding
character levels should be treated as monsters of the appropriate CR and assigned treasure, not equipment. (291)
(Emphasis mine.) And that's insane.
In other words, the Monster Manual wants the DM to compute the monster's ECL—summing its its Hit Dice, level adjustment, and class levels—, then use that ECL to determine where the creaturefalls on Table 4–23: NPC Gear Value.
- Example 1: The jackalwere has 4 Hit Dice, 2 class levels, and an unlisted level adjustment, which forces the DM to give the jackalwere a +4 level adjustment after resorting to the horrible, horrible Level Adjustment Factors from pages 11-13 of Savage Species.1 The jackalwere should have the wealth of a level 10 NPC.
- Example 2: The androsphinx shouldn't have NPC wealth at all, instead determining its treasure according it its CR 18. But if the DM ignored that pregnant generally and gave it NPC wealth anyway, the androsphinx has 36 Hit Dice, 1 class level, and a +5 level adjustment.2 The androsphinx would have the wealth of a level 42 NPC, that is—if my math is correct—about 6,897,000 gp (ELH 317).3
By the way, that "[e]lite vampire, 13th-level half-elf Monk/Shadowdancer" has 13 Hit Dice from class levels and a +5 level adjustment, so according the Monster Manual's own rules—and this should surprise no one—the example should be equipped as a level 18 NPC with gear worth 130,000 gp, nearly twice as much as it has now.
While there's undoubtedly grist here for the killer DM (e.g. a stone giant from MM 124 who's a War1 should have the wealth of a level 19 NPC yet is CR 8, the mind flayer Sor9 on MM 187-8 should have the wealth of a level 24 NPC yet is CR 13), there's literally no other reason to use these rules except to kill PCs using monsters lucky enough to advance by class level.
Despite it being the primary source for information about monsters, I recommend against the Monster Manual's guidelines and, instead, recommend using the Dungeon Master's Guide's guidelines as it's the primary source for information about wealth by level.
1 The Tome of Horrors—Revised apparently lists its jackalwere as having a +2 level adjustment, and this post on the Brilliant Gameologists forum lists the Fiend Folio jackalwere as having a +3 level adjustment. It surprises me not at all that Savage Species computes the jackalwere's level adjustment higher than both.
2 As a cohort. Good enough.
3 My math's probably incorrect, but a level 42 PC has, like, 17.7 million gp (DMG 209), so there's that. 6.9 million gp would be a lot for a CR 18 creature, though (about the combined wealth of 9 level 20 characters).