DEM is a collection of elevation points at regularly spaced intervals over a project area  This collection normally takes place by one of several methods.

Manual collection of elevations from a stereo model or collection from LiDAR point data.  The importance of the DEM is that it is used to create a Digital Terrain Model (DTM), which is then used to show the shape of the earth or control the creation of orthophotography.  The difference in cost from one company to another in the DEM collection varies greatly.  This variation is due to the power of the cost of LiDAR equipment, aircraft and filtering techniques.