How to record prices for the product sales?
What you really should do (and is how a lot of similar type systems architect their schema) is have two normalized tables that store sales information.
One table is the SalesHeader (or sometimes just called SalesOrder) and the other table is the SalesLine. SalesHeader has a one-to-many foreign key relationship to SalesLine on the OrderId.
The SalesHeader stores basic information like OrderId (as the primary key) and SaleDate, where the SalesLine table has a new record for each ProductId with Quantity and Price columns, and its own primary key column called LineId.
This allows you the flexibility to have the same product sold on the same date at different prices, or even multiple products on the same date at different prices, and even batch different orders up (if you wanted). You can adjust the SalesHeader and SalesLine tables as what's more relevant to your use case, but that's the general solution. (Also please don't mind that I use PascalCasing when naming my obejcts. :)