You want painted or unpainted? Metal or plastic? Price range? $0.50 a fig, $1 a fig, $2 a fig, 3 a fig?
Higher quality Halloween party favors should cover any more giant spiders and giant bats you might need. Enormous rats that would need the stats of light warhorses or Destriers can also be had cheaply.
If you don't mind getting and attaching separate bases, the old "Battle Masters" game is seemingly floating around $50-60 online and it has a bunch of Warhammer setting models. Not the best models, but there is a nice mix that should be kinda close to the scale of Descent minis ...
* 3 Lord Knights mounted
* 9 Imperial Knights mounted
* 15 Imperial Men-at-Arms
* 10 Imperial Archers
* 5 Imperial Crossbowmen
* 1 Mighty Cannon with 2 crew
* 3 Champions Of Chaos mounted
* 10 Chaos Warriors
* 10 Chaos Archers
* 10 Orcs
* 10 Goblins
* 6 goblin Wolf Riders
* 10 Beastmen
* 1 Ogre Champion
Auggies is selling the figs from the ravenloft board game at a good price.
Results for Board Games:Castle Ravenloft Pieces
A really cool black orc mini...
abPrices.com - Dungeons & Dragons Miniatures - Howling Orc
Star wars gammoreans are affordable orc substitutes...
abPrices.com - Star Wars Miniatures - Gamorrean Guard
Action figures will make fine giants.
Decent quality dragon toys have seeped back into the market for $20 or less depending what stores you have near you.