The Promote Expensive Products and Promote Lower Cost Products actions signals to the recommendations engine that these types of products should have a preference for the segment of visitors that match the rule.
Assume that you have a section on the website called "bargains" that lists various products that are low cost or are heavily discounted from their original price. A visitor to that section is very likely to be a price sensitive visitor that will find lower cost products more attractive. You can use the Promote Lower Cost Products action to target visitors who visited that section during their visit and the recommendations engine will give higher weight in the algorithm to lower cost products - they will be more likely to be recommended and will appear in higher positions in the list of recommended products.
In the screenshot below you can see an example of a rule that targets these visitors: