Need to track and calculate a changing numeric field

Would I just create a record in a different table every time the value changed in the source table/record?

Yes. Create another table and make sure the webhook adds a new row in that table whenever this field changes. Technically speaking, you could extend this to other fields on the orders table as well. It is your call, though.

Yes, you would create an audit trail child table and record the old and new values.  if there are multiple fields to track, then include in the child table and identifier of the field that was changed.  Like Cost or Sell.

Using an ACTION will allow you to record the old and the new value.  Automations don't yet do the "old" value,.

Thanks for the help.