I thought I had already answered, but I was wrong
I had to use the db to store the old quantity and old price values, because I haven't found a place in the code where I could have the old and the new value of these 2 fields.
Anyway quantity is managed in StockAvailable class.
So I made a module that look for actionProductUpdate hook, for the price, and actionQuantityUpdate, for the quantity, saving the values, that I need to check changes, in the db.
Thank you for your answer!