The average deal is the sum of all transactions divided by the number of consumers who bought from you. Take your total revenue and divide it by the number of sales. If you make $10,000 in sales across 10 sales, your average deal is $1000 per sale.
Your average deal is what your average sale per customer is. Take all your sales and divide it by your sales. This will define your average deal. ie: If you make $10,000 in sales across 10 sales, your average deal is $1000 per sale.