Is it okay for monsters to change taxes or tolls to PCs?
And if so, when?
The idea is pretty simple - some NPCs - monsters, fellow player-allowed races, whatever - set up in an easy access to the dungeon and charge you for access.
It's not a new idea.
It happened in Gary Gygax's Greyhawk campaign, notably here, in Ernie Gygax's "Black Reservoir" expedition. A group of elves let the PCs into an unexplored section of the dungeon in return for a promise to pay them on the way back. (This link is cached here.)
Zak S. discussed a (3rd edition? 4th? I don't know) module set in Greyhawk Castle, and mentioned a clan of dwarves who apparently make their living taxing the comings and goings of adventurers. I commented on it, if you want to read the exchange there. (Short version - he comes down pretty hard on the idea.)
It happens in the Forgotten Realms - one major entrance to Undermountain is in the middle of a tavern, and the owner charges you to use it.*
I've read at least one more instance in an OSR campaign, which I can't seem to find now - NPCs who took over the dungeon entrance and hold it against all comers, but charge you for lowering you into and pulling you out of the dungeon.
It occurs in fiction, too - the Greg Costikyan novel "Another Day, Another Dungeon" features tax collectors at the entrance to the dungeon. Apparently this kind of stuff happened in Dave Arneson's Blackmoor games, too.
But is it okay? Lots of examples doesn't means lots of good examples.
I think it can be okay.
I think it needs the following:
A Plausible Explanation: A bunch of elves hanging out guarding the stairs for some inscrutable purpose? Not bad. A tribe of orcs who live on level 1, and will let you down to level 2 if you really want to go? That works for me. NPCs who seize the entrance to the treasure-hunting zone and let people down and back up? Could work.
A clan of dwarves who apparently make their main living by charging adventurers tolls? Yeah, I agree, that seems weak. You'd need a lot of traffic to justify it. The level of traffic that would entail seems a bit iffy. If the dwarves were part of the city's defense forces and they charged people to enter the city post-adventure, that's fair (albeit annoying).
No believable reason? Bad.
Believable reason (even if it's obscure)? Good.
Avoidability: If the only way into the dungeon, period, is through the toll-takers, that's a problem. It's effectively a cost to enter. Why charge x% of the outgoing treasure instead of just giving x% less treasure?
But what if it's just the most convenient way in? "You can go in past us, or hike 20 miles into the Orclands and go in there. Take your pick." Well, isn't choice a part of the much-loved player agency? It's not a false choice - you can pay for convenience or get inconvenience for free. If the toll-takes actively work against people taking the other choice, that's fine, but not if it means you can't avoid it at all.
Can't avoid it? Bad.
Can avoid it? Good.
Otherwise Surmountable: There needs to be another way besides money to get by. Maybe they'll listen to a good reason (the elves want money to let you by, but they'd let a party of elves skate by free), or have another motive (Kill the evil thing they're guarding the surface against, no charge for you!) or can otherwise be dealt with. And there is always killing them. It's annoying if the toll-charges either have the force of law or the force of moral right behind them. If you're playing AD&D and all halflings are Lawful Good and they're the ones charging a toll, sneaking past them is Bad and killing them is Even Worse. But a group of fellow marauders who set up shop? Chop them into burger, if you can . . .
If it's a couple of ogres who charge people to use "their" stairs, or a tribe of orcs who are willing to let you go fight the monsters down deep if you just give them a taste, or another group you could legally, morally, and/or otherwise effectively deal with other means than money - I think that's fair. I think it's interesting. I also think this works because it allows for other ways to solve the problem - the guards are an obstacle. You can hack them up, sneak past them, or otherwise bypass them - or use money to solve the problem. It encourages negotiation and gives a real reward for doing so (Pay X to be unmolested, Earn X+Y in the deeps). You know your six is guarded, but you can't be sure the guards are trustworthy (will they make you disappear if you come back weak, or aid you?)
So think this can work, and I think it's totally fair. It's just a spin on guardian monsters, anyway - monsters you can bypass with money and who tell you so. It's also just a spin on the magic door with a special key (get key, get entry - no key, no entry), the puzzle entrance, or the hidden entrance. This time the cost is money. The question is, is it fun (avoidability and otherwise surmountable are keys here) and does it make sense (plausible explanation). If it's plausible and represents an interesting challenge, I can't see anything wrong it with. Bad execution doesn't make a bad idea.
* And why are they even letting you in? What if you come back a-running, followed by monsters and/or dripping Mummy Rot? Seems pretty foolish. "Yeah, go on in and rile up the monsters. That'll be 5 silver pieces. Try not to stir them up too much." I mean, the dungeon is under a major city, and instead of dedicating military force to clearing it out and securing the city, they let random yahoos into the dungeon from a hole in a bar floor and hope they don't bring trouble back out? No wonder the realms are a mess. ;)