I've been to Cartagena in October and honestly it's pretty nice - less crowded than summer but still pretty touristy, and prices don't drop that much compared to other times of year. The weather is hot and humid, so get ready for that, but it rains less than in September. As for safety, just stick to the established tourist areas (the old town, Bocagrande, the Getsemaní neighborhood are all well-patrolled) and avoid walking around the outskirts at night, especially alone; most visitors don't have problems if they use common sense. If you don't want to spend as much, look for accommodation outside the historic center, which is where it gets most expensive, and eat at local restaurants away from the main streets.