Two to OneThe house that we're in right now used to be a duplex. In Montreal though, a duplex is one upstairs and one downstairs.
It didn't take a lot to make this into one house. A kitchen had to be removed from the upstairs and a bathroom had to be added. Exterior work had to be done to turn the old double entrance into a single entrance. A guy was here recently scouting our house for a movie set, and he commented that these houses go back and forth from duplex to single family and back to duplex a lot.
Side by side is likely harder because the shared wall is probably thicker than a normal wall, and is also load bearing. You would require the services of an architect and a structure engineer to come up with a plan. As well, it could be difficult to come up with a single family layout depending on what you're working with.
The cost would depend on so, so, SO many factors. You are certainly talking five digits though. I'd imagine that you're looking at $20,000 to $60,000.