Can you suggest a mental framework to keep in mind when asked about listing possible revenue or cost sources?

You want to begin with the simple frame work of thinking about revenue as Operating and Non-Operating Revenue or in accounting terms Operating and Non-Operating Income. The Non-Operating Revenue is almost always the same and includes items such as profits from investments, dividend income, interest income and gains incurred due to foreign exchange.

The Operating Revenue sources will vary greatly depending on the type of commercial transaction that the business is involved with (e.g., B2C, B2B, B2B2C) and the type of business (e.g., restaurant, chemical company, business services company, etc...). You want to obviously start with the core products and services and the associated revenue. But, since the interviewer is pushing you for other potential revenue sources, you need to consider things that are ancillary to the core products and services. Some generic buckets would include the following:

• Core product bundles: Additional core products added on to another core product, often at a discount (e.g., home / mobile and internet, hotel / car rental)
• Add-on products: Complementary products to the company’s core offering (e.g., warranties, extra mobile phone lines)
• Upgrades: Premium upgrades that enhance the core offering (e.g., first class upgrades, premium engine oil, faster internet speeds)
• Partner offers / products: Partner offers that dovetail, but don’t compete, with your products (e.g., Partner Credit Card)