<ex dentist here>
The difference between a general dental practitioner and an endodontist is that the endodontist will have undertaken post-graduate, specialist training in endodontics (root canal treatment) at their own expense and they will also have invested many thousands of £s in special equipment to enable them to carry out complex procedures. This, as well as the surgery time needed to carry out these procedures, is reflected in the cost of such treatments.
The title of endodontist can only be used by a dentist who has completed specialist training and is not, as one ignorant poster suggested, simply a ploy to charge patients more money.
Some root canal treatments, usually in front teeth, are straightforward and well within the competence of a general dentist.
Others, with multiple curved canals, as is often the case with molar teeth, are not.
Hope that helps.