To put it into perspective:
If you charged the batteries twice a day from full-dead, a battery will cost you < $0.16/day.
Figure you probably vape something like 2.5ml/day (an average case), or $1.50/day-ish.
$1.66/day. For me, that's a $4.69/day savings, 140.70/month, $840 saved over the life of a single battery typically.
Just to put it into perspective...