They will usually pay for the anatomy scan it's one of the 3 u usually get but its not necessarily to find out the gender they usually do it to get images of all of the organs and other parts of the baby but if they're able to see it they will tell you I'm glad I paid for mine because I baby was shy at my anatomy scan so they couldn't tell me so good thing I already knew !
Everyone is different and it also depends on where ur placenta is located and where the baby is also located at that time too because I found out at 15+6days but they also checked to make sure the gender was right so they reconfirmed that it was 100% a girl when I was 20+1day