How can the value of a field be set to be dependent on that of another field?

For reporting purposes I want to assign a numerical weight to each issue priority.

For example:
Low = 1; Medium = 10; High = 25; Critical = 25; Blocker = 50.

However, I don't want to have to rely on user to independently set this value.
How do I create a field called, for-example, "Priority Weight" that takes a value dependent on the chosen value of the "Priority" field?

It possible to approximate this behaviour using the custom field type "Cascade Select" but the values in the two select list appeared concatenated by a hyphen on the issue navigator. I want the "Priority Weighting" to appear in a seperate column - so that it can be summed.