In Scotland, salmon have been preserved by salting and smoking since the 13th century, a process known to the native North American Indians of the North Eastern seaboard even earlier. Precisely when the lightly cured and cold-smoked fish that we now call smoked salmon was first produced is not clear, though it was probably as recently as 100 years ago. These would have been fish prepared specially for people who had caught them by rod, a service that some smokers still provide.
The quality and price of what you buy varies enormously, wild smoked salmon being much more expensive than farmed. You can buy both as whole sides to slice yourself or in good delicatessens you can have any amount sliced to order. However, most is bought ready-sliced and vacuum-packed. You can pay anything from £10 a pound to £18 a pound and what value this represents will be as much to do with your personal taste as any objective criteria that may be applied. How long the fish is cured, whether the cure includes ingredients other than salt, like sugar or rum, and the type of wood used to burn for the smoke will all affect the way it tastes.