It would help to know what other tables and relationships are involved here. Is this a pricing table linked to both a table of products and a "LineItems" or "InvoiceData" table used to list the products sold on an invoice record?
Let's assume that you have this relationship now:
LineItems::Product = Prices::Product AND
LineItems::Level = Prices::Level
You can then add a calculation field to Lineitems named cNextLevel defined as: Level + 1 with a Number result type.
You can then add a second Tutorial: What are Table Occurrences? of Prices and link it in as:
LineItems::Product = Prices|NextLevel::Product AND
LineItems::cNextLevel = Prices|NextLevel::Level
and then Prices|nextLevel::Price will return the needed upgrad price.
In FileMaker 12 and newer, a Calculation field could also use ExecuteSQL to reference this price without the need to add another table occurrence to your relationship graph.
I think I understand but I'm going to read your table occurrence post and make the example db so that I can get my head around it.