Cost of downtime

Answer:
Downtime consists of two types of costs:
1. Direct, like:
- Lost profit
- cost generated by resolving the issues which caused downtime (hours of your employees)
- SLA penalties (or even lost SLA)
- cost of en-user (un)productivity
. costs towards third parties
- etc.
2. Indirect, like:
- reputation damage
- cost of lost opportunity
- regulatory/compliance breach costs
- etc.

So, as you can see, while calculation costs of downtime some costs are easily identifiable, and some are just good estimation. Financial people or business will be of great help, so use the chance and include their inputs in calculation.