Capacity Planning - Measure and compare peak and business hours requirements for Memory, CPU, Disk IO and Network Bandwidth. Peak for some systems may be during off hours while backups and ETL processes are running. Connections and batch requests per second may also reveal potential issues.
Security - Security concerns may dictate what can be consolidated.
Is that reporting instance really required? Applying and maintaining proper indexes may eliminate the need for ad hoc databases. Performance tuning can have a significant impact on costs.
I believe SQLCAT produced a white paper on consolidation.