it probably would be cheaper booking this year..they probably use the same system as some Budget Airlines. the futher in advance you book, the cheaper the fare seems to be.. I'd recommend if you know the dates you want, book them asap.
This has been the case for several years, but here is a cautionery tail.
I booked a ferry crossing in Sep 03 (£190) for a sailing in August 04 at the time the price was very good. someone in our group did not know for definate that they could come so they booked in July 04.
I had to change my booking as I had changed motorhomes, strangely my booking came out far dearer than the person who booked late (£160), when I questioned this I was told because mine was an advanced booking the replacement booking had to be done at the rates in force when I originally booked ! and I could not take advantage of the by now far lower prices !
the only person in our group of 10 vehicles that booked late also got the best price by far !
Its a gamble, booking in advance at least you know what the costs are now, they may be cheaper by next year, but will not be dearer. However if they are cheaper you will not be able to take advantage.
It is a strange one and as never happened before (booking clerk said) , so the odds are stacked in your favour.