Good basketball lenses are the 85/1.8, 100/2 or 2.5, 135/2 when shot from the floor under or near the basket. On a crop frame body the 50/1.8 you have would be a good lens, but you'd have to get in as close as you can since 75mm isn't that much. 1/500th @ wide open @ iso 3200-6400 is about what to expect in most high school or college gyms. Anything slower than 2.8 will not work.
Nighttime football is pretty dark. 1/500th @ f/2.8 @ 3200-6400 iso is pretty average for most high school or college games at night. 200mm on a crop body would be about as short a lens as you can get away with.
Even for a day game you want to shoot sports wide open, since that gives you a shallow depth of field to help isolate the action. For under $500 bucks, I would look for a used 200/2.8 maybe. That would be a good lens for baseball (infield at least), football and even basketball indoor when shooting at the opposite basket. Use the 50/1.8 you already have for under the net stuff.
And this is all assuming you have reasonable access to the court or field where the game is being played. Down where the other photographers are sitting. Shooting from the stands will make your job a lot harder.