I've been using the promoted gigs feature since its release and although I don't have any sales through it marked yet, I do see people landing in my inbox through promotion.
The daily budget you'll set would rarely be fully expended, but still feel free to set a daily limit you'd be absolutely comfortable with! You can treat it as a cost that you are "ready to risk/lose" as there are no guaranteed sales. Also, if you are not using proven free methods to promote your gig then it doesn't make sense to spend money to do so, at least imo.
I personally go with $5 to $10 per day and it usually ends up charging me between $1 to $3 a day, and the variance is huge when it comes to different gigs - some barely get any views, while others would expend even 90% of the allocated budget, so it all depends on what people are searching for Live it seems.
I hope this helps. ^^