BTW the goal is not to use the balls forever, ball savers don't keep the fuzz on the balls. If you play on hard courts and play hard and with lots of spin, no good. If you don't hit that hard, or play on clay, they are a great option to double or triple the life of the balls.
In my case, our outdoor season in Canada lasts about 6 months. I play every day and usually use new balls at about $5 per set of 3 for ProPenn with tx. My opponents vary from 4.0-5.5, so I only look to double the ball life.
That's 180 x $5 per season in savings. The ball saver let's me use the same balls twice at maximum pressure, so I save 90 X $5 = $450 per season in savings, while still playing with premium balls.
Over 5 years alone that's $2000.
Current & recent affairs: PS97 (enjoying), ai98 (sold), Extreme Pro 2.0, Steam 99s, PK QT 295, Dunlop 3.0T (still like a lot) & Bio 400T