Welcome to the Asana Community Forum @Pedro_Mendonça!
You can utilise custom fields to enter the value such as the amount and the quantity, however at this stage calculations are not really possible yet. In list view sums of average can be displayed already, but you cannot multiply yet. Here is an existing feedback request to upvote.
You should be able to achieve this with tools such as Asana2Go
Now in terms of automating the follow-up can you clarify what exactly you want to set up?
You can definitely achieve a lot by setting up rules and you can even send
emails via Gmail triggered by rules.
Does that help?