Mostly you're buying a higher quality device - better fit and finish. It used to be that only the expensive 'pods had swivel tops, but now there are plenty of less expensive units that do (and still some that don't...). Spending reaches a point of diminishing returns though, where you can end up paying a lot more for a small improvement. In a few cases you're paying for a name (and maybe import-related costs), e.g. Parker-Hale. The expensive ones are usually more robust, but I think most guns lead a pretty gentle life, so that may not actually be that important.
GsT