I would say don't get an octopus if you can't afford to get it everything it needs. I mean that in the most respectful way. It may seem expensive, but that's nothing compared to what you'll have to spend feeding it, replacing equipment it breaks, keeping perfect water quality, and unexpected expenses. Additionally, you should set aside $300 in case he breaks your tank, and you have to run out and get a new one. There is no chiller that is under $250 for a 50 gallon that will work longer than a year. I have been an aquarist since I was 4 years old and can tell you that going cheap always ends up costing you more in the long run. Sometimes the animal pays the price. Please reconsider your pet choice or choose a warm water species. My last piece of advice is that you could look into used chillers on eBay or offerup. Or perhaps wait for them to go on sale at the end of the year?