Let me see now. Twenty hours flying cost between one and four thousand pounds according to what you own or hire. Two batteries which will work for around that time less than one pound. Get a life, people.
And the whole battery thing is a pain in the bum to be honest they always seem to go just as your on approach. And if your headset goes down in the first sector of 6 your feel noticeably more tired at the end of the day.
Its not really the cost per say its just the logistics of always having the thing working.