Better to compare it to a cleaning service. Say $30 an hour, and the roomba does the rough equivalent of an hour a week when you let it run daily. 4 years (208 weeks) of $30 is over $6000, ten times the price of the 780. Even if the roomba does the equivalent of 6 minutes of maid-time each week, that's not bad.

Not even close - cleaning lady can do a lot more than just vacuum the floor.

>Say $30 an hour

Depends on where you live. Around here (CE Europe) I get cleaning service for ~5$/h (~15$ for 3 hours of work once a week).

Youd have to pay for extra time for services besides vacuuming. Even if your price is 6x lower, the roomba is still cheaper.

But the cleaning lady doesn't take nearly as much time vacuuming as a roomba I just don't think it's a comparable service at any level or that it can replace a cleaning service - it can supplement it by vacuuming more often.

Minus the time to do it yourself? 20 minutes @ $40/hour * 52 weeks ~ $700 year.