you should already have a "price" parameter in your family?
Assuming this is the case, in your schedule you need to add the "price" parameter, and the "area" parameters to your list.
(Im assuming its £X per m2?) Change your Header in the "price" column to read "Price per m2"

Then you can click on the little symbol that looks like an "fx" in the middle to create a formula.
Name it "Total Price" now click on the little symbol beside the formula box and put (Area/1)*price
You need to divide area by 1 to remove the m2 otherwise you will get an uncompatible units error.

Go to formatting, and on "Total Price" select "calculate totals"

Now you just need to add your price value into your casework and it will calculate the rest for you.

Some prices, especially early on, require a length, not an area (ex base counters of x depth are y amount/linear meter)

You need to figure out which dimension you need (think about which dimension you use when doing it by hand) and make sure you can get that number. Putting the price in cost and multiplying it times the length in the schedule as Karalon mentioned would be the simplest way to do it