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has the best prices online generally, though the quality can vary greatly from reviews I have seen.
If you are on a budget, check craigslist in your area. You can often pick up tanks for dirt cheap that look brand new. If you go this route, you want to pay less than $1/gal
Petco and Petsmart tend to have recurring $1/gal
sales throughout the year. You could ask your local store when they might be having the sale again.
The filter cost is going to very greatly, as you alluded to. It depends on the type, the brand, the size, etc. etc. If you are on a tight budget, make sure you are being realistic with what will be required- there is a lot more to setting up a new tank than just the glass and the filter. Consider - salt, heaters, refractometer, test kits, cleaning tools and supplies, food, livestock, substrate, live rock, lights, etc etc.