well, a special-plus costs 3 euro on the German website. That's for Germany. Outside Germany, prices are usually higher. So a special-plus would probably be 3,50 to 4,00 euro on f.e. Playmobil's Belgian site. And on the site, sets are usually cheaper then in most shops... So let's say 4 to 5 euro in european non-German shops.
More accesories are nice if they are something exclusive or if you're just buying 1 or 2 specific specials, but if there is a special-plus you want a whole bunch of (like a soldier or knight f.e. to establish a regiment/army), it will cost lots more then today with the normal specials...