I've done a Bookbub twice in the Teen category, which has a much smaller list size than Romance (and thus costs a lot less).
Both times, I made back the cost of the ad and then some (roughly double both times) just on the day of the ad, and the increased sales continued for a couple of weeks (albeit nowhere near at the same pace as the day of the ad) afterwards.
I've gotten sales figures from several friends in the Romance and Mystery categories, and I have not heard of a single person yet who has not made back their investment. Many have done exceedingly well. Some have even hit the "big girl lists" like USA Today or NY Times as a direct result of their ad.
In general, it seems that 1500-2000 sales the day of the ad alone is a rough average for Romance and Mysteries (of course, some sell less and some sell many, many more). Of course, the more venues you offer the deal, the better. Usually it appears that authors make back the investment and then some through Kindle sales, and then it's B&N and Apple that really puts them over the top.