I'm in the states and I'll assume you are too, so your milage may vary
You'll need a 29g+ tank with hood and lighting, so maybe $60 there
You'll need live rock (20lbs ~$130) / livesand (~$20 depending) / crushed coral sand for buffer ($15?) for tank furniture
Octopets sells an octopus bimaculoides (a species suited for aquarium) for $40 w/200 clams for food (their OctoPets Kit), and I think shipping on that is $25 but can't remember... so $65
Oversized hang on tank filter (rated at 60g maybe) $40'ish, and protein skimmer (I got the prizm hang on tank skimmer for $70 + shipping from windowtothesea.com which seems to be the cheapest I can find) so $80'ish
We're up to $225 for tank, $65 for the octo, $120 for heavy filter/skimmer which will not only help do a lot of filtering, but will add a lot of flow, so that brings you up to about $410
The only things you would add would be like a backing for the tank ($10 max) and maybe duct tape to seal it all up if you don't already have it...
Other things you will need will be a thermometer ($2), hydrometer ($12), salt mix ($20 for a 50g bag I think), a bucket to mix stuff in ($5) and I guess maybe a $5 net.. $5 syphon tube for water changes... -- I'm assuming you aren't brand new to aquatics though so maybe you already have some of this But if not that's probably another $50 or so
$400-$450 might sound like a fair investment, but for every last thing you're going to need and several months of food and the fish, I think this is actually not that bad!
We are recommending a fifty gallon tank for a bimac - that makes it a little more expensive. Anyone considering an octopus should read the Ceph Care articles carefully - Colin's Equipment List gives a lot of good information.
The are ongoing expenses for an octopus. You'll need to buy RO/DI water or purchase a special filter unit for aquariums. Even if you use frozen shrimp, it's still expensive, and your octo also needs some live food.
You need to test your water frequently, so you need test kits, and to replace these kits when they run out. You'll go through salt pretty fast, too.
Since there are a lot of options, depending on your choices and what's available to you, you'll have to price it yourself. Some people spend $500 for the initial set up, others $1200. By all means visit and LFS and have a look at the complete range of aquariums.
Ongoing expenses are higher if you don't live near the ocean and have to mail order live food. Food expenses are less when your octo is small.
I would imagine you'd spend at least $20/month for water, salt, carbon, test kits, etc. for a 50 gallon tank. Food could be more.
Having a Prism Skimmer for my reef tank, I would reccomend getting the deluxe version. It adds a pocket for carbon or whatever you wanna put in it and a surface skimmer attachment for it pulls the waste from the surface of the water. I noticed a dramatic improvement because of this feature.