creating a custom field in ms project

Hi,

I have a resource say A and B whose prices are 10 and 50 dollar respectively and there unit is kg.

Now I want to create an resource C whose unit is sq meter.

1 sq meter of C = 0.9 of A and 0.5 of B

so now if I use say 1 unit of C my cost will be calculated as below,

cost = 0.9 * 10 + 0.5 * 50

So how can I create resource C in ms project?