var google_conversion_value = 15.0;
The 15.0 represents a $15 cost of conversion, entered in by the advertiser.
The cost of conversion value can be attributed to how much a product costs, how much each lead or sale is worth for a company. You can learn a lot about a company with this metric. The question is, why isn't this metric encoded some how? An oversight?
Excellent find and excellent thread, forum discussion at WebmasterWorld (requires paid subscription).