Check the latest articles that are made in this area .. You will find alot of papers dicussing pricing strategies .. One suggestion that you can choose one/some strategies and measure its/their effect on consumer (think about one variable related to consumer behaviour)
In general I agree with Hajar Alhosseiny. In more detail, you could work with some pricing models that evolved due to digitization like Freemium in specific contexts (e.g. music/movies or industry). Another interesting topic could be the free-to-play with micro transactions model that is often used in gaming (especially mobile apps). There are also certain psychological factors involved like triggers to become addicted. However, when you choose a topic make yourself familiar with the state-of-the-art literature surrounding the pricing topic as suggested earlier by Hajar Alhosseiny
Explore the research area first. Previous research on pricing and marketing will help you to define your topic. Pricing strategies or pricing and consumer behaviour in digital world.
i quite agree with Hajar because that is the way of doctoral study in order to scholarly add to knowledge and fill existing gap. Meanwhile, you may look at area of Positioning and Individual Disposable Income relationship