Do you actually see so much change in the salt levels week-to-week that weekly testing is necessary? If you were testing monthly, the breakeven point would be about 7.5 years; I'd be skeptical that the tester will last that long. Pool professionals are presumably doing such tests on SOMEBODY'S water multiple times a day, so their experience doesn't really translate to someone caring for a single pool.
Of course, if you really need the weekly tests then the payoff time is under two years and it makes more sense.