I've found that even for my RPG games, that some of the minis designed for WFB and LOTR SBG are really cool. For example, one box of Empire free company militia will have you sorted for any thugs, footpads, citizens, sailors, bandits, coachmen, inn patrons, and even PCs if you paint one well enough - its a bout the best value pack of models you'll buy! Some of the hero models are quite cool too!
Another quick way to fill up a cavern with greenskins is to buy the LOTR Moria goblin sets. Cool looking gobbos, I like them more than WFB, they are less cartoony. Other enemies which are fought quite a lot are beastmen, best to buy a box of plastic WFB beastmen or Gor units than individually picking through metal figurines one by one...
In a way, buy one box of the ‘fellowship’ and that is most RPG parties right there! Good ol' archetypes!
what I'm getting at is that there are some box sets of cheaper plastic minis out there now which could satisfy most of your needs with just a few purchases.
And remember..zombies and undead are just all your spare models, with some arms and stuff cut off and painted to look like ghosts :D lol
...what models do you use?