You can certainly save money owning your own tanks, even when you factor in yearly inspections and 5-year hydros (which really aren't that expensive or inconvenient).
I pay about $11 to fill an HP steel 100 with EAN with a fill card. That same tank would cost me $21 to rent at the same shop. So, I save about $10 a dive. An HP steel 100 cost me about $400 last year. I can offset the cost of the tank by diving it 40 times.
A visual inspection is about $25 a year and a hydro works out to about $14/year if you spread the cost out over 5 years (70/5). So, about $40 a year in tank maintenance. You can offset maintenance cost by diving it 4 times a year.
Using this math, I've paid off both tanks and saved around $900 in tank rental costs this year alone. YMMV depending on the type of tanks you own, how often you dive, and where you dive. For my circumstances, it makes sense to own to my own.
Further, I don't like to worry that the outfit I'm diving with isn't going to have the type and size of tank I want. Also, at least one shop northern PBC where I live will not let you choose the mix you put in a rental tank (they put 34% in all rentals). I like to have the flexibility to use the most optimal mix for the dives I'm doing.