Winger - you should be able to buy those tires on the net for anywhere from $245. to $257. shipped to you (and even lower on some sales), so you can figure what your dealer is charging you for installation. You should be able to do a lot better.
Personally, I'd question you on why the Dunlops? They would be my last choice on the GL1800 and for the overall return on the investment and satisfaction you would expect from a Goldwing tire, I would consider either the Avons or the Bridgestones. If you can get the newer Metzler front then consider the Metzlers. You'll pay slightly more for the other three, but overall they each are better tire combinations (IMO).
Do some research, talk to friends who own Goldwings "and" have driven a lot of GL1800 miles to get some firsthand experience. Also watch your presssure religiously. It's a controversial topic and lots of different experiences with each tire brand so we all have our own (good and bad)opinions on tires