Ask HN: How can I automate the monitoring of conversion rate on my website?

Has anyone figured out a way to receive an alert if the conversion rate on their website drops by a certain percentage? I already use Gomez to ping my site and execute synthetic transactions, but executing every permutation that could identify poor performance can get expensive. If my average conversion rate is 1% and in the last 30 minutes that dropped by 50%, there is a good chance something is wrong and I could limit the impact by starting my investigation. Has anyone written something custom for this or found a product that supports this?