My vendors update about once a year. Some products stay in place, and others are removed, while others are added. What pricing would apply? Is that the $250.00 per vendor (some have less and some more than 50,000 products)

No, the charges are $0.01 per product. So it mainly depends on the count of products being added. Since there is a promo price of $50 for every 10000 products, so estimated to be $250 instead of the usual $500.