As you can find here Best Practices For Horizontal Application Scaling, vertical scaling is much more expensive than horizontal scaling:
vertical scaling cost increase exponentially whereas with horizontal scalability cost is linear.
Why is it so?
Vertical scaling means "adding more power to existing machines", while horizontal scaling means "adding more machines".
In my opinion, upgrading existing machines should be cheaper, as some parts already exist and you only have to buy additional processors, RAM, etc., whereas buying whole new machines should result in higher costs.