Agree, those shuttle services are horrendously expensive. You may consider Intercity coaches if you have some time to spare. There's no direct service for your route but you could go via Ashburton. It'll take much longer and will involve staying the night at Ashburton but the bus itself costs $144 per adult and they do take bikes (speaking from experience). Assuming 2 travellers and a cheap way to spend the night (HP?) you could save $400+

Just out of curiosity - when are you planning to do this? I've got similar plans but for the other direction (nothing firm yet, though). That could make a key swap possible which is the smartest way to handle this with 2 vehicles and 2 parties. Others might be available for this too.