Finalists are granted access to the API on Feb 11, 2013 and are required to submit their entries by March 1, 2013. Assuming that they also notify you as a finalist on Feb 11 and not before, that gives you 3 weeks to build and submit your prototype.
$25k / 3 weeks == $8,333.34 USD per week. That's a pretty good payout for anyone in my opinion.
Also, assuming that you're not spending 40 hours a week at this for all 3 weeks (120 hours). A working prototype can be made in 10-15 hours. Also the amount of time you spend on it is up to you.
So 1:30 shot at $50K, for a 15 hour investment? Up to you to decide.
It can be a goood deals for CS students that have some 3 weeks mandatory project to do for school.