I actually kept an octopus a few years back, an O. Bimaculoides.
I named him Ozzy, and he was one of the best fish i have ever kept. I think he cost me around $35 and he was great. I trained him (by trained, i merely mean put the obstacle in his way) to open cork bottles, unscrew jars, and other little things to get his favorite food of ghost shrimp. He was a great fish for a species tank and had great personality. He would sometimes be incredibly stubborn and grab ahold of my hand while i was doing tank maintenace or whatever and wouldn't let go for probably a half hour. He even made friends (i swear, he did) with a yellow tail damsel who guarded him from nippy little feeder killies.
He was a great cephalopod and well worth the investment, even though he only lived about a year.