How much should I charge?

for my a2 computing project I am creating a custom online room booking system that teachers log on to with their credentials and are able to book various rooms around the school. They have said they would like to buy it off me once I have finished and I am just wondering how much I should charge? As this will be my first time selling software. I don't want an exact figure just a range

Main features - Login system - Alternating weeks - Able to view rooms they've booked - Contact IT support about problems

~ Many thanks Reece Oliver