If you are a student or teacher, this is the only true "cheap license" that exists.

If you are currently enrolled in an academic institution, this is by far the best option available.

Maxon One Student License: Eligibility, Application, and other FAQs

Finding a "cheap" license for ZBrush is challenging because Maxon (the developer) has transitioned almost entirely to a subscription-based model. However, depending on your status as a student or your specific workflow needs, there are a few legitimate ways to lower the cost. Legitimate Low-Cost Options