Infrastructure as a Service (IaaS) typically requires the greatest up-front expense by the customer when migrating a data center to the cloud. IaaS provides virtualized computing resources over the internet, where customers rent virtual machines, storage, and networks. The customer is responsible for managing the operating systems, applications, and data. This model often necessitates significant initial investment in planning, migration, and configuring the infrastructure. In contrast, Software as a Service (SaaS) and Platform as a Service (PaaS) models usually involve lower up-front costs because they offer more managed services.
Reference: CompTIA Network+ Certification Exam Objectives - Cloud Models section.