It's pretty simple all you need to do is create two trigger like this:
Trigger 1
Events
Unit- A unit begins casting an ability
Conditions
Ability being cast equal to <YOUR ABILITY>
Actions
if life of triggering unit greater than <The spells blood cost>
then
do nothing
else
Game text message display to owner of unit triggering unit the text: <An error message like Not enough blood>
Order triggering unit to stop
Trigger 2
Events
Unit- A unit starts the effects of an ability
Conditions
Ability being cast equal to <YOUR ABILITY>
Actions
set life of triggering unit = (life of triggering unit - <BLOOD COST>)
put your actions here.
I know that the functions aren't totally spelled right her but thats how to do it.
The first trigger stops the unit and displays an error if the unit hasn't blood enough, the 2nd 1 takes some life from the unit and does your actions.
Hopefully that helped you...
- Blade.dk