We rented a car a couple years ago and visited Perast, Our Lady of the Rocks, and Kotor. It was really great all around and I would recommend it. We enjoyed the unique setting of Our Lady of the Rocks, and had a nice lunch in Perast. Kotor is very interesting, but it was broiling hot when we visited in August.. Driving and parking were pretty straightforward except for one close near encounter with a bus on a narrow street we probably shouldn't have been on. Our car rental was under $50 via Hertz at the peak of tourist season, so I think you can do much better than $180, but it'd be great with a tour, too. No problems or terrible traffic crossing the border either way.