brand new carbon fiber tank, 90 cubic inch, I think called a "Buddy Bottle" that also includes the regulator set for 3000 psi, hoses, valve and everything you need for filling your air rifle for $300. That's what I use, and get 8 or 10 refills for a Marauder. Pyramyd Air also has the huge tanks for $550. I'd say that the $300 is top end that you should expect to spend.
Remember that if you buy a used scuba tank, you'll need the "fill station" kit with valve and hose. eBay sells those relatively inexpensively, Brand new is around $80 on Pyramyd.
After looking and looking and looking, I finally bought the 90 cu inch from Pyramyd for $300 and was done. It included the tank, 3000 PSI regulator, and fill station hose and valve. No guess work. Used tanks on eBay are often close to or over their 15 year life span. Be careful, as after 15 years, they are not legal to refill. For $300, you get a full life span, and 5 years before your first hydro test.
Good luck on whatever you decide.