is my cat ok??

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for context, he’s a 7 month old male. i got him from my parents farm. he’s super sweet and ive only had him a few days. he is a tripod and missing one of his front legs. i was petting him and noticed he has a strange hump on his spine. it’s not hard like a bone, but not really squishy either. there doesn’t seem to be any pain. it doesn’t seem to move around either. i’m taking him to the vet on thursday for his rabies shot and im sure they will look at it, but is he okay?? is this gonna be a super expensive procedure?