I am working on a spreadsheet, which calculates $$ spent on cigarettes over the years.
This question isn't so much as how to write the formula (at least not yet), but more about - what is the proper way to calculate this.
When I started smoking 21 years ago - I was paying $2.50 per pack. When I quit smoking on 11/03/13, I was paying $10.00 per pack.
I first got my average of those two values, which would be $6.50 - but is that the value I should use to calculate the cost of cigarettes?
I understand this value won't be 100% exact, because we have no idea how much a pack was for each year I smoked.
So what is the proper way of calculating this, knowing how much they were when I started - and how much they were when I quit?
Thank you in advance for your help!