D&D Minis tend to run between $1.50-$3 these days for basic PC and monster types you'd want, with the more iconic large monsters (dragons, beholders, etc.) closer to the $5-10 range. They're simply the best out there for getting a lot of decent quality minis for cheap. The sculpts vary widely, so make sure you to look at a good quality pic. Personally, I tend to order from RPG Locker and Troll and Toad most often, depending on if I need stuff other than just minis and who has the better prices for the specific stuff I want.
If you don't mind unpainted minis, the board games like Castle Ravenloft are good for getting a start to your collection (especially that one since you don't really need to paint a skeleton much). If you're like me and don't have a steady hand or artistic talent, it might not be worth the time or trouble of painting them though. And personally, I have a collection of D&D minis at this point and mixing painted and unpainted minis sets off my OCD tendencies.
If you want to go a bit cheaper, other game minis work. Mage Knight tends to be the cheapest, but Monsterpocolypse (or whatever it's called), Heroscape, HeroClix, and Star Wars Miniatures all work too, but each have their problems.
Star Wars is the same scale, but if you don't take a REALLY close look at the minis, you might end up with a wookie you thought would've made a perfect bigfoot that turned out to have a laser pistol.
The newer Heroscape minis work perfectly (again, watch for hidden guns on the sculpts), but the older ones have a flared edge to the base that makes them just a teeny bit larger than 1" bases.
Mage Knight and HeroClix both have the hidden anachronism problem in addition to a base that is just way too big for the 1" scale of Dungeon Tiles and most game mats. The figure scales are a teeny bit off, but not enough you'll notice. However, they can flat-out be the cheapest options to buy in bulk (either singles or as a lot on eBay) and it only takes a little bit of work to make them fit.
If you have a mini that's on a base that's too big for your grid, you just have to cut off the figure and glue it on a new base. Take an exacto-knife and cut the minis off the base, then use wood glue or epoxy (superglue) to glue them onto a 1" base. If you don't want to pay Warhammer prices for plastic bases, look around for unfinished 1" wooden circles for crafts. Dip them in some black paint and they're perfect. You can also do this with the thick cardboard WotC uses for the tokens you're forced to buy with friggin' EVERYTHING these days, but the glossy print they use on them makes it hard to keep the figures glued on like they're supposed to (if you paint the tokens first then glue, the paint comes off the gloss...but if you glue the figure on first, you have to paint the base around the mini).
When searching eBay, don't forget to use one of the typo-catching search engines. Sometimes you can luck out and catch someone posting a 200 figure collection for under $100 because they posted it as "Dungoens and Dragons" and it doesn't show up on normal searches.
If you're going to go through the trouble of getting minis rather than going with tokens, make sure to get some terrain. This is where Mage Knight really shines in D&D. You can get cases of the walls, towers, etc. for dirt cheap on sites like Troll and Toad and they work perfectly for D&D. If you've already got the exacto knife and you have a color printer, get some cardstock paper and use some of the printable terrain. Some of these don't work well, but the stuff like staircases and walls are easy to assemble and work amazingly well. You can also get wooden blocks in 1" cubes to use to add height to Dungeon Tiles adding a 3D effect. If you find the right site, this can actually be cheaper than buying foamcore and far easier than cutting it. Sorry, I'm just a huge whore for 3D terrain and feel if you're using 3D minis on 2D maps, you're missing out on a lot.
The granddaddy of good hauls for terrain (and unpainted minis for that matter) is the old HeroQuest board game. There's a reason the thing does for $300 or more on Troll and Toad when they have a copy. I've seen them on eBay going for under $50 though, so that definitely might be something to look into. Believe me, the furniture ALONE is worth the price, let alone the minis (again, lots of them like skeletons and mummies that don't need to be painted).
