Cloud is not automatically expensive. On-prem is not automatically cheap.
Compare the same service level
A useful model includes hardware depreciation, colocation or facilities, power, bandwidth, spares, support, staff, security, backup and disaster recovery. Cloud cost includes compute, storage, data movement, support, observability, managed-service premiums and committed spend.
Model uncertainty
Do not compare a fully redundant cloud architecture with one server. Model a low, expected and high case for demand, hardware life, staff time and migration effort.
Value the option
Elasticity has real value. So does predictable cost, offline continuity and control over replacement schedules. The cheapest line item may not produce the lowest business risk.
The point: the answer changes by workload. The calculation should expose that, not sell one platform.