The deal with B&Bs kind of depends upon the area. In lots of places, they do cost more than an average motel room, because they offer more ambiance and a real breakfast. But in some places, they really can be competitively priced with chain motels, and are often nicer places to stay.
In the city I live in, even a cheap chain motel will run you $100-$120 per night, sometimes more. When my parents visit me, they pretty much always stay in B&Bs, because while those also cost $100 per night, you get a heck of a lot more for the money.
So it's definitely worth looking at B&Bs, particularly in areas that have a lot of them (since that can be a sign that they are competitively priced with other lodging options), but others are right that they are often not particularly cheap.
But in general, I'd say that roadside motels cost on average $100 per night, with variance based on location, region, and time of year.