Bob, forgive me but I'm not understanding your formula at all.
First, what do you apply this to and/or what is the unit of area? Is it printing? Do you apply your formula to cut vinyl, banners, dimensional painted
signs, etc.? I'm assuming that your formula doesn't include materials, substrates, etc. since you seem to add them at the end but would that be the case for something like printed banners, which I think would be a prime candidate for your formula?
Second, where is the sliding scale? Your formula for your example will always be $24 for a unit of area:
($24-$6)+$6
($18)+$6
$24
This is how I would apply your formula using a maximum rate of $3 and a minimum rate of $1 for the digital printing and $3 for the banner substrate cost:
1 banner 3'x10'
($3-$1)+$1*30+$90
($2)+$1*30+$90
$3*30+$90
$90+$90
$180
10 banners 3'x10'
($3-$1)+$1*300+$900
($2)+$1*300+$900
$3*300+$900
$900+$900
$1800
This results in the same cost for 1 banner as in 10 banners, tell me what is wrong please.