There is no way of knowing if a sale is a result of a contest view. Many times the host of the contest will twitter and facebook the winning images , which will certainly drive more traffic to your image, but whether or not those people looking at the social media posts are buyers you will never know.
IMHO, it only takes a couple of clicks to enter contests, so I do it. It's not a big time-eater, so I do it. If it took longer than a couple of minutes, I would not bother to enter contests. Mostly I like to enter to see other artists work.