If this line:
Product lifetime value (£100 - provided as a metric from business) X No. of individual sales
is read as:
"100 pounds multiplied by the number of individual sales"
Then that £100 is a static number, and you can include such static numbers in calculated metrics.
First, click the "+Add" button to the right of the calculated metric builder, and choose "Static Number":

Then, enter your static number. Remember to change the operator from the default division to your required multiplication.

Hope that's what you're after! 😀