Using Polynomial Equation from Trend Line

Morning guys, I have a problem I'm stuck solving!

See attached for example worksheet (the yellow cell is the one to change and freetype).

Basically, I have 3 known values (sq meter) and an associated cost. When you enter a free type sqm in the yellow cell, the result is a value from the polynomial equation along the trendline of the known £ values.

My problem is this - if I were to increase the known values to a set amount, or by a percentage, the polynomial equation fails, as does the result as the formula is now wrong!!

Is there a way to actively use the poly formula so that it can update other cells if the formula were to change?? I use a similar system for other areas to the 5th power, but now face the same situation whereby I need to increase the known amounts!

Many thanks guys,

That's amazing Kaper, thankyou. I've heard of linest but was never sure how it can used in my own work. Is this easy to expand to 5, 6, 7 known values? Is it just a case of adding them in, or would it amend the formula (=D4*A8^2 +E4*A8 +F4) for example?

Thanks again