Hasbro did a D&D themed line for their Kre-O line. It didn't do too well, sadly.
Because competing with Lego in the block based build-able toy department is super hard.
So you just have to hunt down fantasy, knights, Lord of the Rings, and Harry Potter sets. Or go with knock-off Lego. Chinese sites like aliexpress.com are a great source for that.
Kre-O was horrible... I generally wouldn't even look at knock-offs, but I guess they are ok if on a strict budget.
With real Lego, it's generally expensive to buy complete sets on the secondary market, because of the "collector's effect". More reasonable deals can be found on spare minifigs, but it actually depends a lot, for some reason certain minifigs cost a fortune even if used. Generally speaking, any
licensed set and minifigs (like those you mention) become unreasonably expensive on the secondary market. If you really want to go that route, a good website is
www.bricklink.com
Personally, I would suggest to keep an eye on
new sets currently on the shelves. Interesting monsters can be found in any fantasy theme, for example Ninja Go or Exo Knights. Just don't go looking for a perfect match with D&D monsters
Also, don't forget to check out the "collectible minifig series"! In the past they have been a good way for us to get multiple copies of monsters such as werewolves, vampires, minotaurs and even medusa, as well as pick one for each fantasy character to gather PC parts and weapons. Unfortunately their price has now increased to 3e per minifig over here, but we still pick the occasional minifig (for instance, in the latest series there is a Dragon Suit Guy that makes for a good
dragonborn character, if you just turn his face around or replace it with a black one).