Wc3:WoW is pretty much completely free. Only thing you really have to pay for is WarCraft III: Reign of Chaos and WarCraft III: The Frozen Throne, since you need both of those two games installed in order to play Wc3:WoW.
It is illegal for anyone to force anyone to pay for their mod with just about any game, including WarCraft III. In fact, Blizzard Entertainment has shut a few other maps and mods down in the past for putting a price on their projects. Wc3:WoW respects Blizzard Entertainment's Terms of Use, End User License Agreement, and so forth. We're pretty much helping them out with Wc3:WoW, we think, since it will eventually become good enough to the point where players who have never tried WoW but have played Wc3:WoW may actually consider playing World of Warcraft instead.
World of Warcraft is hands-down always going to be better than Wc3:WoW. The WarCraft III engine is extremely limited and the fact that it is single player makes the development of this mod even more difficult to make the mod fun and interactive. Nevertheless, our goal is to make this one of the most entertaining single player RPGs ever created for WarCraft III. It's already the best WoW replica mod, but what we've got so far is hardly enough, in my opinion...
Anyway, in short, Wc3:WoW is a free WarCraft III: The Frozen Throne mod.