I'm a bit of a cheap ass and would barter with them if you are going to purchase that amount of weight. It never hurts to try.
As it stands, doing a search on the web, Ivanko itself charge 68.3 cents per lb for its cheapest "black economy" plates. The standard grey are a bit more expensive even. As you pointed out, that doesnt include tax or shipping, which will probably eat you alive.
Like others have mentioned, I know dumbells sell for about 50 cents a lb, but I have never really researched plates.
I didnt follow the link you posted, but highschools usually dont have shoddy benches so I will assume that it is a good one...$100 good I dont know. You can do your own shopping there.
I can tell you are looking for someone to tell you it is a great deal to get these plates as fast as you can. If you dont have to pay tax, I would buy them. If you are going to drop $200 anyway it might as well be for Ivanko plates at another plates price (even though name brand doesnt mean much to me). I would assume the plates are already chipping, etc if they are anything like my highschools were and I dont know how "straight" this high school is running the sale, but I would try to barter a bit more still. Twenty bucks less is still twenty bucks.
BUY THESE PLATES!!! There does that make you feel better? A month after you get them it wont matter if it was the best deal or not, you will just have a super set of plates to get huuuuge with.
My three cents...now thats a bargain.