MVC SelectList combining multiple columns in text field

You could create a new anonymous class using a simple LINQ projection, and then use the SelectList(IEnumerable, string, string) constructor overload to specify the value and text fields to be used for the <option> elements i.e.:

var stands = 
  db.Stands
    .Where(s => s.ExhibitorID == null)
    .Select(s => new 
     { 
       StandID = s.StandID,
       Description = string.Format("{0}-- £{1}", s.Description, s.Rate) 
     })
    .ToList();

ViewBag.StandID = new SelectList(stands, "StandID", "Description")

Edit

In C#6 and later, string interpolation makes for better reading than string.Format

   ...
   Description = $"{s.Description}-- £{s.Rate}"

If you project to a strong ViewModel class name (instead of to an anonymous class), you will undoubtedly want to replace the magic strings with the safety of the nameof operator:

ViewBag.StandID = new SelectList(stands, nameof(Stand.StandID), nameof(Stand.Description));