I'm going throw out a couple of comments from the perspective of a backpacker and a bicycle tourer. I really don't think you have to go with absolute top quality when bike touring. Sure you want something that will keep you warm and is reasonably light and compatible, but it's a little different than getting a sleeping bag for backpcking when your survival might well depend on its reliability. In most cases, if you end up in a compromised situation with your bag while touring, the worst you're going to experience is an uncomfortable night as you'll most likely be able to make it to a town or find help the the next day where you can get shelter, dry out yourself and your gear and warm up, etc. However, in the back country, that might not exist and you could find yourself in a real survival situation. Been there, done that. Therefore, I don't think you necessarily have to invest hundreds and hundreds of dollars in a sleeping bag.There are plenty of good bags out there in the $200-$300 range, some less, that would suit you well on a bike tour. Of course, if you're touring in really remote areas or in extreme climates, then it's a different story and you might want to invest in a really good bag. Long story, short, it's nice to have a really good bag but I'd buy what you can afford and what's appropriate for the journey as long as it has some credibility. I'd stay away from department store and box store sleeping bags and would definitely go with down.
I always keep my sleeping bag inside my panniers in a waterproof sac. Gives me a bit of peace of mind.