Go into data/spells/spells.xml then you find the name of the rune you want to edit
For Example im using a Magic Prison rune here is what it should say:
XML:
<rune name="Magic Prison" id="2272" charges="1" maglv="20" exhaustion="2000" blocktype="solid" allowfaruse="1" event="script" value="custom/magic prison.lua"/>
Find
and edit it to whatever exhaustion you want.
Done.