Simplest way is just calculate the value and assign the result to the field as you fill it. The SetField method of an AcroForm object takes only strings as the parameters, but if the calculation is simple, you can do an inline calculation then call the ToString method of it. For example
Code:
'somewherer in your code, you have the quantity of a product store in a variable named "quantity"
acrFrm.SetField("product.totalcost", (3.00 x quantity).ToString)
Otherwise, you can always calculate the result then set the filed value separately.