The costs listed would be for a single implant, just as it has always been. Pairs would be double the cost, unless part of a cybersuite. As for why you'd get spurs on both hands, there's plenty of reasons I can think of, mostly springing from the rule of cool, but dual-wielding blades that can never be disarmed is a good one. And there will probably be more ideas that come to you as the 5E version of Arsenal comes out.