In general, B&Bs are likely your best buy, particularly considering that the price includes a full breakfast. Most B&B breakfasts are big and you can often do without lunch. B&Bs with private or ensuite baths generally run anywhere from $75-100 depending on location and other amenities. If you don't mind a shared bath, you can likely get a B&B for anywhere from $60-90 again depending on location and amenities. There are some motels/hotels in Sydney that you can get from $50 up but they won't include breakfast and you may find that some of the lower end ones won't particularly meet your needs.
If you do want a private bath, keep in mind that your term for private and ours is different. For us the term "ensuite" is what you call private - ie actually part of your own room. For us "private" simply means "for your use only" and it will be next door or acrosss the hall. It will not be accessible directly from your room.
If you decide what type of accommodation you want and what you are willing to spend, I would be happy to recommend something appropriate. It is still "high season" in Cape Breton until the end of our Celtic Colours Festival 18 October.