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!
Good luck and feel free to ask for clarification
Robert