Students looking for a script writer

We are a second year film students looking for script to produce for student film competition, we have the idea but unfortunately there isn't a writer amongst us to bring it to life. Being students, we really don't have a budget for a script nonetheless we don't expect things for free. How much would you do it for and how long would it take you? the script should be about 15-20 minutes in length. we have the idea but needs developing.