Considering that these ideas (if this challenge was meant to be taken seriously) would be part of a likely dozens-of-billions-of-dollars project, $5,000 seems like a deliberate insult.
If the roadmap contained a clear vision for manned missions to Mars followed by permanent colonization, they wouldn't even be bothering with public input.
It's a PR stunt, with prizes that fit into a PR budget. Note that the announcement says nothing about NASA doing anything with the winning submissions.
NASA has done -- and continues to do -- great things with robotic exploration of Mars. But they are hamstrung by politicians: both the ones that hold offices in their organization and the ones on Capitol Hill who hold the purse-strings.
The NASA of today doesn't do things today because they are easy, but because they will look cool and funnel money to the right states and congressional districts. Full stop.