Determine two changing variables only knowing the result So, about a decade ago my company came up with pricing for some banners that we sell. the prices are as follows.
$43.68 for a 3x4 banner
$44.52 for a 3x6 banner
$46.36 for a 3x8 banner
$50.00 for a 3x10 banner
$52.54 for a 3x12 banner

and I can not figure out where these prices came from. The guy who wrote them up quit before I started, and I need to figure out the equation to extend the pricing up and down.

Here's what I DO know.

The equation is based off two things
The cost of the banner per square foot

The cost of labor 

I do not need to figure out the factors that went into pricing for either, I just need to know what numbers they are.
Best guess for labor was 63 dollars, it might not be, but if that works, it sounds good to me.

my attempt was to figure it out using substitution with a system of equations.
12(sqft) * X($/sqft) + 63($/hour) * Y (hours) = 43.68 and
18x + 63y = 44.52

with a second set of
24x + 63y = 46.36 and
30x + 63y = 50.00

BUT the first set gives me 
x=0.14
y=0.66667

and the second set gives me
x=0.606667
y=0.504762

which leads me to believe that the hours per banner change. Meaning the y in each equation is different. Is there a way to determine what these two variables are, even though one changes, probably linearly? If not, I'll just do a whole new equation, the only issue is the number of variables going into each of these variables.
Thanks.

If you use the formula .4*area + 38, you'll get approximately what the current prices are.  It's an easy formula to apply if you want the new prices to seem consistent with the existing prices.  However, if your new sizes include dramatically larger areas, then spend some time investigating costs.
