Can I boost or bury results with a Rule based on a condition like less than or more than?

Currently, there is not a built in way to achieve this without adding an attribute to your records. 

Why?
Boosting or burying records (using optional filters) requires a specific value, which rules out a range or any conditional logic. 

This article describes why you can't do this and shows an example: https://support.algolia.com/hc/en-us/articles/5851934555025

Is there a  workaround?

Sure! You can create an additional attribute on your records which holds a specific value. For example, if you were originally trying to boost t-shirts with price < $20, then you could add an attribute 'cheaper_shirts' with a boolean value to each of these records, then use that attribute in your Rule as it is now a specific value.