Add additional information to a field of a content type
Some solutions you have:

1) create field Product with multi values to content type Order bases on a table field. Module : https://www.drupal.org/project/tablefield

In this table you can attached the prices and with a custom module do the counting.

2) Create content type Order, create content type Product. In content type Product add a field price. Make a relation to entity from Order to Product.

3) Make use of ubercard. This will define products, cards and as far I can remember the recalculation of the card can be refreshed easily

4) Make use of Drupal Commerce, I know it is possible in there but never used it.

Let me know if you need more details.