Cephalopods are an extreamly difficult marine critter to care for, and when you manage to get one that actualy stays small (a few breeds) they tend to have extreamly short lifespans (just the way it is not poor care) from around 6 months to 2 years, and if you get a specimin that has been an adult for a wile, then its entirely possible its lifespan will be almost complete and live only a few weeks more.
So in conclution they are an extreamly expencive critter for something that will not live very long, thought at the same time they are one of the best marine pets you can own as they have a ton of character. But again not a beginner spiecies nor is it to be steped into lightly. So if you are really interested go here, http://www.tonmo.com/, they have everything you will ever need to know about them, but again I cannot stress enough its not something to jump into they can be very hard to keep and short lived. So do your research.