In "The Case for Mars", Robert Zubrin explains plans for a mission to Mars that would cost one time development costs of around $30 billion, and $2 billion per mission after the development costs. That could put us to having a manned mission to Mars, without increasing NASA's funding substantially, within a decade.
Sending a manned mission to Mars will have similar economic effects that the Apollo mission had, in increasing interest in STEM fields. It will also allow us to better understand Earth, and much better determine if Mars has life.