My standard rate for doing such an assignment is a flat $125. For that I guarentee at least 30 images and no more than 50. One condition that I also list is the kid must play enough to get enough images. If the kid has a bad game and you don't get her hitting a basket, well thats not your fault. You can only shoot what happens.
To be on the safe side I also make sure I get a few sideline shots of the kid too. Get em during a timeout, or when there is a dead ball. You can usually get a good sportrait with a great expression if you watch close enough.
I did this for a 7th grade football player who was supposed to be a pretty decent player. Well that game he was anything but a great player. No tackles, no interceptions and as a running back he only carried the ball 1 time and that was the first series of downs. I had one shot of him heading into the line and the rest was only legs until he was tackled.
I did however have several good shots of him chasing down a runner while on defense, a couple of sideline shots, and 3 or 4 of him during halftime when the team all took a knee in the endzone as the coach talked to them. One of those shots he was looking dead at me and had one eyebrow raised. His mom loved that one. As a finish I did 2 shots of him and his mom after the game with the lights and field in the background. She was happy and I think she had 40 images to choose from.
During the 2nd quarter a dad approached me and asked about my services. His son played on the 8th grade team which immedaiately followed. He asked how much I would charge to shoot his son, who was a lineman. I told him $125 and he asked if a check was ok. Between the games he came back and handed me a check for $125.
Total time, about 3.5 hours.
On shooting sports...If you see it happen then you didn't get it.