Listing all the products from table A could be as simple as 'show All records'. But you may not really mean ALL records, but all records for a particular company, which can be done via a single button click or selecting that company from a drop down menu. Either action can perform a script that finds all records. These can be presented in a list view with a field next to each product for recording the new price for that product.
There are details to this process that depend on your data model--the fields, tables and relationships set up in your database so I can't really provide much more than that without knowing more about your database design.
The Customers table is linked to the main PriceEntry table which in turn is linked to the Product table. The Product table is there to provide the list of end product in order to generate the entry list on the layout. The PriceEntry table exists to accommodate every product's price against every customer.
So what I'm trying to achieve on the PriceEntry table is something like:
PKID | ProductIDFK | CustomerIDFK | Price
1 | 1 | 1010 | 2.99
2 | 2 | 1010 | 4.50
3 | 1 | 1012 | 2.79
4 | 2 | 1012 | 4.51
Hope that makes sense!
This is a join table linking records in product to records in customers. A product can have many customers and a customer can have many products.
So you can set up a portal to PriceEntry on a layout based on Customers. The ProductIDFK field can be setup with a Value list of product ID's and product descriptions so that a user finds a customer record and then starts selecting Products in the ProductIDFK field. This is the "beginner level" method and I recommend that you set this up as phase 1. When this works for you, we know that your relationships and layout design work.
Then, if you want better ways to find a particular product, you can use one of the methods illustrated in "Adventures in FileMaking - Enhanced Value Selection" to produce a faster, more user friendly interface design.