If you want to see the step-by-step guide to create a new Log Analytics alert, check out our recent blog post on creating Log Analytics Alerts.
For the alert signal logic, use the following values:
Use the query from the previous step
Set the sum of AUs to 50 as the threshold (you can use any number that reflects your own threshold)
Set the trigger to 0: whenever the threshold is breached
Set the period and frequency for 24 hours.
Read the whole thing if you use Azure Data Lake Analytics; an unexpectedly large bill is a tough thing to swallow.