In 2012, the NGUI programmer stated that NGUI was making him three times what his previous job had paid, and that his starter kits were making $700-1500 a month.
http://www.tasharen.com/forum/index.php?topic=2126.0
So that suggests there's certainly some revenue there.
NGUI was also one of the most successful plugins in the entire Store, so his revenue, while significant, should also be treated as a ceiling on what a company can expect to get.
I think revenue from the Asset Store is relatively minor compared to that. Ive heard of success stories that sell plugins worth a couple of thousands per month, but thats about it and those are not many.