It's simple. A brand with established guidelines and imagery usually gets an hourly quote from me as there is not too much room for creativity. If I work on designing an expo booth for the same brand, the price is per project and hourly as it requires a lot of imagination and a complicated space to work with,a lot of changes, a lot of people involved and also many limitations. For new brands or designs I quote a price per project. The fees depend a lot on the project or client (it could go back and forth for weeks or just an hour of vectorization of a client's drawing -yes, although it is not ideal, I also like these type of projects, because ultimately and especially for small businesses, branding is a very personal matter-) Here are some indicative design fees: