It could cost anywhere from £0 to £500 depending on the quality of wood you require and where you source it from. You could build something using leftover planks by skip-diving and taking a trip to your local tip. Or you could buy nice fresh packs of timber from B&Q, but doing it that way you could probably buy a whole shed for less.
Difficulty would depend on your experience and tools available, but since you had to ask the question I'm going to say it would take you at least a few weekends to finish it (assuming you're aiming for something as well-built as the one in the picture).