It sounds high just for replacing the belt and a spring but did they do anything else???
I just put a belt on a friend's mower. I worked all afternoon getting the belt tighteners, linkage and idlers to work correctly. He is a big pressure washer guy. His stuff always looks like a million dollars but his maintenance is lacking. He will wash and wax but would not dream of oiling a pivot bushing. So things stick and don't work after awhile. If I had been charging him full shop rate he could easily had a few hundred dollars in getting just a belt replaced. I also had to fix what made the belt go fail.
I would ask the dealership for a itemized work order. Then asked them what all was done and at what cost. You picked up the mower and did not ask what all had been done. Now your condemning the dealer just based off a MAILED bill.
I like the guys that are all dumping on the dealer on this. Telling him to never go back and all types of things. Any of you guys ever heard of COMMUNICATION!!!! Call the dealer and asked why the bill is what it is. THEN if the dealer just says "well we just replaced the belt and that is all." you can dump on him.
If they took the deck clear off the tractor. Cleaned it all up and sharpen the blades. Reinstalled the deck with a new belt. Adjusted the belt tension and leveled the deck they could have easily had an hour or two involved.
So find out what was actually done. Asking some Cub mechanic on a forum to tell you what the bill should be is kind of not the way to resolve this issue.