In Jira On-demand, can I set the value of a field based on two other (Custom) fields?

[marcdeutsch]

I have two custom fields in my sub-tasks: Qty and Item Price.
I need to auto-populate a "total cost" field on the sub-task, and on the parent item, which would be a sum of the sub-tasks.
Is this possible in On-demand Jira? 

[Jobin Kuruvilla [Adaptavist]]

Nope, not possible in onDemand.