Make unit testing part of your core development methodology, and ensure that test coverage is part of your "Definition of Done." Also, ensure your estimates include the overhead to develop and refactor your unit tests.
How to "Bake In" Unit Testing
How can I make unit testing a priority?
Unit tests can function as both design tools and quality controls. As such, they should be included in your project's "Definition of Done" for each feature or potentially-shippable increment. By baking the unit testing into each increment of your work, you ensure that you aren't stuck doing a lot of post-facto testing at the end of the project.
Prioritization can be handled by making explicit tasks or milestones for the delivery of unit tests. However, this isn't really a very agile approach to the problem. Instead, you would be better off padding your estimates to provide time and budget that can be allocated to the ongoing development and refactoring of unit tests throughout each project's lifecycle.