Welcome to tfl!
JohnKSa covered it pretty well.
There are, essentially two different values possible. The value of it as usable ammunition, and the value of it because of what it is, meaning rare/collectable/historical value, if any.
For collector value, the condition of the rounds and the condition of the packaging matter. A box of WWI era .45acp ammo (with the correct box) is worth a lot more than the box of .45acp you bought at WalMart last week.
The first thing you need is a comprehensive inventory.
We'll be glad to help identify anything you can clearly describe, (pictures are a huge help) there is a very broad knowledge base here at TFL.
Get a copy of Cartridges of the World (Barnes). Older copies can be found pretty cheap sometimes, and are a pretty good general reference for old cartridges, like obsolete black powder rounds. IF the ammo you are dealing with has any batches of mixed ammo, probably loose rounds it ought to be looked over.
Over the years, a lot of us "brass rats" wind up with single rounds or small numbers of rounds in odd, unusual, even rare and valuable calibers. Sometimes these just wind up in a box, or a bucket, with other stuff. usually other ammo we can't use...and often the cases don't have regular headstamps identifying it.
I'm not into cartridge collecting enough to be up on the current prices, but I do know that there are rounds rare enough to be worth a lot to certain collectors. A single rare round might be worth $75? $100? I don't know, but there are rounds in that category, and sometimes, they've been known to hide out in a bucket of mixed "scrap" brass/ammo.
If you've got anything like that to deal with, you really need someone who check it over, someone who can see if there are any diamonds in the mud.
Good Luck, we are here to help if we can.