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If you are going to buy a tank for inverts, it is always better to purchase a brand new one, as a used tank may have had copper medications in it before (will kill an octo in no time ).
Most of the stores have sales often on 50's and 55's...typically about 75-115 bucks for the tank and a lid. From there you have to purchase all of the filtering merchandise you are going to require. A conservative estimate is about 400+, with most costing around 700...be sure you can afford to do it !
I think you'd be better off to wait until you've decided on all your equipment before buying the tank. Greg is right, you'll probably have to buy another sort of filter and may use few of the extras that come with such a set up.