Generally drum scanners are of an impressive size, and correspondingly expensive, costing around as much as a car or a small house. These are used by pro labs to scan your film, so it is hardly surprising that a high resolution scan can cost $50 to $100 a time. The drums used are typically a metre or so long, and a number of images are scanned at the same time, each negative being held on a small part of the drum's surface.
The light source is in the middle of the drum, and the photocell outside. As the drum rotates, it scans every point on a circle around it. The light and photocell then move a fraction parallel to the axis of the drum for the next line to be scanned. The curved drum keeps the film to light source distance constant, and avoids problems with reflections.