Decreasing cost per square foot..?

Hi All,

This is my first post so please forgive any posting protocol blunders I may make, the first one being that I don't know which topic heading to post this to.

My question is this, if I make a sign at 8ft x 4ft and charge £200 for it then that is at a rate of £6.25 per square foot, which is fine. But if I use that square foot rate to calculate the cost of a small door sign at say 1ft x 5ins it gives a price of £2.60 which is way too low as it should cost £15.40.

Conversely, if I use the new square foot rate calculated from the new cost of the small sign, which is now £36.96 and use that to calculate the cost of the 8ft x 4ft sign it works out to a ridiculously expensive £1,182.72.

It's obviously not a linear increment related to a standard value per square foot, but we all accept that the more you buy the cheaper each individual unit becomes. Can anyone help me derive a formula that allows a calculation that reduces cost per unit as the number of unit purchased increases?

I enjoy the odd mathematical puzzle but I'm not getting anywhere with this one......