With my first son I found out at 15 weeks, it wasn't an elective U/S, it was one ordered by my doctor. This one I had one at 13 weeks, I didnt even ask if they could tell cause I knew it was so early, found out at 20 weeks.
A lot of early U/S sex determination is wrong though, so keep your receipts at least till its confirmed at your A/S.
Personally I wouldn't spend money on an elective U/S, but I've had 4 U/S already, I might feel different if I only had 1, or if they couldn't tell the sex when I had my A/S.