Given the lack of details in your profile, an easier question to answer is
should anyone buy their own tank?
Here are a few situations where tank ownership could be recommended;
- A diver who needs bigger than 80cft
- A diver who lives and dives far from any tank rental location
- A diver who needs to be able to dive on a moments notice
- A diver who dives doubles
- A diver who just has to have either HP or steel tanks but lds doesn't rent either
Here are a few reasons most divers just rent;
- Annual visual inspection cost ~$15/tank
- 5-year hydro inspection cost ~$25/tank
- Initial cost of a tank ~$100 (used) - $300+
- Low cost of tank rentals vs air fills
- Air transportation issues
For example, If I bought 2 used 80's for $200 (7 mo vip & 3 yrs hydro remaining) my 3 yr cost is ~$300. If I get 200 fills at $3/fill in those 3 years that brings my total cost to $900. If I had just rented at $5/tank my total cost for those 200 dives is $1000, so it makes some financial sense. On the other hand, if fills are $2 and tanks are $3 (fill card or tank card) it's $700 vs $600 with rental being cheaper.
With regard to tank rental cost, you can not just compare that to the cost of the tank, you have to consider fill cost also. In the areas where tank rental is high, is fill cost high as well? Here in the Islands it is not uncommon for local divers to pay in advance for 250 fills, or 250 rentals, but either way the difference is only $1-$3. Nitrox is another story; my lds charges $10 for any nitrox fill and $10 for any nitrox tank rental, so renting their 100cft w/EAN36 is the same cost as filling my 30cft w/EAN32.
Financially, regular used tanks (63's & 80's) could pay for themselves in a few years / few hundred dives. New big tanks could take 10 years / 1000 dives to pay off, but if you are in any of the top catagories, cost may not be your only decider. With regard to flying, the valve needs to be removed for air transport, neccessitating a VIP before use at your destination. Full size tanks also may be charged as extra baggage. I only know of some researchers and imagers who transport pony tanks by air, then sell them to the locals before returning home.