The cheap ones only tell you where you are
the expencive ones tell you where your going and how long till you arrive !
Cost of the unit is only part of the issue.
Available maps ? what year where they last updated
Topo mapping ready ?
Nautical map ready ?
Size of memory
Global map included ? if required
ease of use, brightness and clarity of screen.
Horizontal or vertical use
vehicle mounts ?
Hard wire cables
USB ?
Service centers ?