Most of the time when you purchase something for your convenience you
pay twice. Once to purchase it and once when you make use of that
convenience. Typically, the first cost is measured and the second one
is ignored.

Take 24/7 email access for example. You can get it using blackberry and the costs are typically:

Cost of the unit: $X
Service fee:       $Y per month.
Total:               $X + ($Y times the number of months)

Actually, there is another cost which I think is easily ignored. Every
time you are playing with your son or daughter and in the middle of the
play, you take some time out to check your emails and probably respond
back to one or two of them, it costs. It’s just that your son or
daughter is paying for it!