If you need savings to justify buying tanks, here goes:
Renting tanks from a dive shop often means two trips, one to pickup, and one to return. If you dive 30 times a year and the dive shop is 20 miles away, That's 1200 miles of extra driving. That's like going to Florida and back for some of you guys.
Tanks rent for about five dollars more than an air fill.
Also... a tank has the highest resale of any diving item. Don't abuse it, and you can sell it for good money in 10-15 years. Would anyone pay much for a 10 year old wetsuit or BC?
If you want to show your wife that you are thrifty and don't throw money around, buy a couple of tanks.