Welcome to the Boards, by the way.
Besides eBay, I recommend
www.maxminis.com - There you can buy, sell or trade miniatures. Maxminis also has a list with prices you can use as orientation.
Decent size dragons: The problem with that is that they are among the more expensive miniatures: You're probably speaking about Large Red and Silver Dragons and of course Huge Red and Gold (I got a huge gold in a booster - the only rare Huge I got out of a booster). But at least with the large ones, you might be able to get them for less than 20 bucks (go got them for less than 20 €, too)
demons or devils: To be frank, there aren't that many of them, especially the two Big Ones are missing (and will be for quite some time, as their stats would be so far beyond the Skirmish Scope, but I think they should do them nonetheless, and to hell with skirmish ;-)). Dretches, Quasits, Hell Hounds, Abyssal Maws, Abyssal Eviscerators and Lemures are non-rare, and therefore not too expensive. Bearded Devils will be a problem, because they're Harbinger (which is out of production), but Erinyes, Bone Devil, Vrock should not be that expensive, and Glabrezu isn't as cost-intensive as the Dragons.
giants: Cloud Giant and Fomorian are uncommon huges, and therefore not that bad. Storm Giant is rare, but it was the rare huge that cost me the least (if you don't count the Huge Gold, which I got out of a booster). Then, we also have hill giants, which are probably less than 20, too.
orgres: Look for Ogre Zombie and Ogre Ravager - the Ogre is from Harbinger (again, OOP and therefore more expensive).
Trolls: again, Harbinger. Try the Ice Troll
Beholders: So far, we only have the Gauth, anyway. The real Beholder's coming in march.
Minotaurs: Look for the redone one in Giants.
Giant spiders: That one confuses me: It's quite high in price for something you can get in every toy store.
For your convenience, I have looked up the maxminis list prices for the minis (note that these are prices for direct shopping. You'll probably get better prices when you trade or buy them via maxminis or ebay - I have paid a lot less for most of those):
Large Silver Dragon 21.59
Large Red Dragon 22.48
Huge Gold Dragon 34.98
Huge Red Dragon 39.99
Bearded Devil 19.99
Bone Devil 12.09
Erinyes 9.53
Vrock 12.35
Glabrezu 18.65
Hill Giant 21.23
Cloud Giant 4.79
Storm Giant 17.31
Fomorian 5.42
King Snurre 15.99
Frost Giant 25.50
Fire Giant 21.98
Ogre Zombie 10.09
Ogre 27.31
Ogre Ravager 12.78
Troll 25.98
Ice Troll 16.24
Gauth 12.45
Manticore 12.89
Owlbear 21.50
Minotaur (Harbinger) 16.04
Minotaur (Giants of Legend) 8.75
Minotaur Skeleton 8.03
Large Monstrous Spider 12.64