Whats the difference between purchase prices and cost price in Sumtracker?

Cost Prices in products are different from Purchase prices.

Cost price determine the actual landed cost of each product. Cost prices are used for stock valuation and calculating the gross profit reports. Cost price is automatically updated when you actually purchase an item. You can also enter the cost of products directly. There can be only one cost price of each product.

Please see this guide to learn about how to update cost prices and how cost price is calculated when you receive products in a Purchase Order.

Purchase prices are the prices you have negotiated from your supplier and intend to buy your products at this rate. They are used in making purchase orders. There can be multiple purchase prices for a product, since it can be sourced from multiple suppliers and can have different purchase rates from each one. 

Please see this help guide for how to create and update purchase prices.