HI wacky,
We have a lot of good information in our Ceph Care articles to help you get started: just click on the Ceph Care button above.
The cost of the initial set up can vary a lot - I think the latest suggestion was about $20/gallon for everything.
Likewise the cost of keeping an octopus - do you live near an ocean and can catch some crabs and other food for your octo, or will you have to buy it all, including some fresh crabs for snacks? They learn to eat thawed frozen shrimp, but that's not inexpensive and it may be worth it to buy good quality. Water and salt costs will vary. You'll need to make a shopping list and see what costs you come up with. You'll also need to buy test kits for testing your water.
So start reading, and also look at past posts on here on Ceph Care. You can also search for specific topics.
Good luck!
Nancy