There is nothing to purchase at all if you want to train your own realnet models on the CPU such as Pedestrian, Car, etc. Just follow the documentation at https://sod.pixlab.io/api.html#sod_realnet_trainer on how to do so.
What are the differences in capabilities? Not sure yet. I'll have to look more into it, but I was hoping somebody here could tell me if they have experience with both.
Why would I use this over darknet and the variations of Yolo - YOLOv3 or YOLOv4?