I’m no longer a collector (been out of that stuff for years), so consider my advice in that context.
* If you’re looking for basic monsters, DON’T buy the random packs! You’re unlikely to get what you’re hoping for, so you may end up with lots of minis you don’t want.
* Stores and online often sell singles from the random packs. Sometimes the price for one mini seems steep, but it’s cheap to spend $10 or $20 on one mini than hundreds of dollars hoping to get it randomly.
* WizKids has “battle packs” (or whatever they’re called) with a handful of related minis together. Great for getting a band of skeletons or goblins or whatever. Unfortunately, those packs are like $50 for 5 or 6 minis, which is still a lot if you’ve got a big collection yet to build.
* WizKids also sells TONS of nice unpainted minis for all kinds of monsters and heroes (D&D, Pathfinder, MTG, and maybe even Critical Role are all licenses they have minis for, and they’re all on the same scale). These are cheaper and you can get exactly what you want, but you have to paint them yourself—some people really enjoy that, and others hate it.
* Other brands of unpainted minis exist; Reaper, Games Workshop, etc. Price and quality ranges wildly, and sculpts may not match the artwork in your D&D books
* BEWARE: You will never have enough minis for all common monsters! You will always find yourself wanting more of them. They will begin to take up space and eat up cash, and you’ll find you never use most of them. These are the reasons I dumped them from my life.
* Consider generic minis, such as chess pawns or wooden meeples. You’ll never need to worry about whether it looks quite right when they’re generics, you’ll spend far less money, and you’ll get more use out of each.
* Consider tokens or standees; tokens are flat cardboard, and standees are cardboard attached to a lil plastic base or folded to stand upright. These are cheap, light, easy to store, and have art printed on them. D&D has official tokens, and Pathfinder has official standees, so exact artwork matches are possible! Or print your own with EXACTLY the art you want! If you lose one or need more, no big deal, just print a few more. (I do this, it’s want more economical than real minis. Also I glue my homemade tokens to wooden circles from the dollar store craft section.
* Jellybeans, coins, and dice are all classic techniques too. Dice have built-in HP tracking functionality, and jellybeans give your players a tangible reward for every kill.
Hope this helps.