From the question it sounds like this is more about forecasting than it is about tracking but I could be incorrect on that. If I were tracking something like this for a client I would generally track a conversion when any item is purchased and then I would track the conversion value as the dollar amount of the actual transaction. You could also leverage Google Analytics e-commerce tracking to get good, detailed numbers on the items purchased and amounts of each item and of the total transaction.

Based on this logic your logic and a conversion rate of 2% you would have 2 conversions out of 100 visitors and a totally average sale value of $x where $x is the total value of the conversions added up and then divided by the number of conversions. If sale 1 was $50 and sale 2 was $100 then you would have a 2% conversion rate with an average transaction value of $75.