Really, you people havn't heard this yet http://kotaku.com/5061980/starcraft-ii- … -a-trilogy ? Though someone was gonna post it sooner than me but here ya go,
They are gonna split up the singleplayer in to 3 games, starting with Terran then Zerg and last Protoss. Multiplayer will be avaible on all DVDs so even if you just buy one of them you will be able to play with the others.

But if you are making a Custom map for SC2 and want some specials units from Zerg campaign you need to buy that game. They havn't told about the price yet so don't get so upset, then we might have to wait a year for each campaign. So during that time I'm quite sure you can make about 50$ or less for the game.
Also the campaign will be as big as 30 missions, the total amount Starcraft was without Brood War. So each race will have 30 missions. Then if you are thinking of pirating the game... who is the cheap bastard now?