For context, if you get the beefiest MacBook Pro right now you'd pay an extra $1000 to upgrade from 48GB to 128GB RAM. For lower end machines you pay more for RAM, $200 per each 8GB you add on top of the 8GB baseline.
But that's if you want Apple to do that for you. What if you buy the memory separately and put it in yourself? I'm considering doing that for my own iMac, and I saw that a 32 GB memory unit is about $65, so 4 of those $260. For laptops it's probably different, but surely it's going to be less than $1000.
You can't anymore, none of the Apple Silicon Macs have upgradable memory. If you have an older Intel Mac then it might be possible depending on the exact model.
Can't add RAM to any Mac (except the Pro, I think).
You have to buy it from Apple with all the RAM it'll ever have. Which is why they overprice the upgrades so much
You can't add RAM to the M series Mac Pro. It's just a Mac Studio with PCI-e slots these days.