They’re actually really common to run into on mystery islands. They are popular though. I think Marina and Zucker would be like 10-50 NMT. Octavian too.
I actually have found all but Octavian multiple times. I've found Marina five times, Zucker three, and Octavian once.
They're not rare, every villager has the same likelihood of appearing.
They are actually a little easier to find due to there being only three octopi.
Like others have said, they're not particularly hard to find, since there's only 3 of them and the game chooses species first before the villager. I've seen Octavian many times, while bumping into Marina and Zucker a few times as well. They are pretty popular though, so I definitely think you should search around for the one you want.
I've met all three during my island trips, with Octavian the most frequent (3x). So yeah, there's a higher chance of meeting them compared to species with a large pool (dog and cat, for example).
But price-wise, I've seen people do them cheap for 10-20 NMT. Or if you're lucky enough, they even give them away for 5 NMT or even free (because as mentioned above, they are more common).
I think that if you save up some NMT you're bound to find one of them because everytime I go island hunting I run into one of them every time. Like others said they roll the species first then the villager so only having 3 of them means they're more common than other villagers such as cats and bunnies that have more villagers in their species.
it all kinda depends on which octopus and who you’re buying from. i’ve noticed that zucker and marina are usually more expensive meanwhile octavian is on the more cheaper side of a price range