Well, firstly many congratulations on your upcoming wedding.

From my many many work trips to north Queensland, I can confirm that some
good places to stay would be, in this order:
(1) the Sea Temple Port Douglas. But do not stay here if you are only in a studio room. There is a huge difference between the studio rooms and the one-bedroom apartments but you should be able to get a one-bedroom apartment for the amount you are willing to pay;
(2) Peppers Bale Port Douglas, but this may be more expensive than Sea Temple;
(3) Sea Temple Palm Cove;
(3) The Shangri la in Cairns in the Horizon Club; and
(4) Sebel Harbourlights in the event that the above four are too pricy.

I suggest you try to stay in Port Douglas or if not in Palm Cove as these places are much more romantic than Cairns, although Cairns can be very nice.

The way pricing works for Cairns and north Queensland is that the prices tend to come down for accommodation the closer you get to the dates. Try www.wotif.com, also try the Qantas website to package the accommodation with airfares. But since you are looking at October 2010, have a look at the pricing available now to give you an idea of the pricing available for the same time next year.