I don't think anyone has enough info on the land to give you an educated guess. There are many things that go into determining the value of the land -- Is the lot flat or on a hill? Is there a view? Is the lot able to be subdivided? Do other homes close by have similar sized lots, or is this the only one that's this large? ...and so on.
Just like a with a building, there are a lot of components to consider when calculating land value...your agent should be able to run comparable sales in the area that are similar not only in house size, but lot size. I'd ask him/her first...