The Merlin pass is currently on sale with Individual passes reduced from £169 to £119 and Family passes reduced from £129 per person to £99pp, allowing up to 3 adults to be on the pass. Premium and VIP passes are available for extra - click the image above to compare costs and benefits. I couldn't actually see when the sale ends though.
Barks Web Pages
Alton Towers - Merlin annual pass sale
Lots of the families that stay at Barks Holiday Cottage use their annual Merlin passes to visit Alton Towers. One family stayed at Barks three times during the autumn as they were especially keen on Scarefest. I think they got value out of their Merlin pass!