How to soundproof music practise rooms

I’m Head of Music in a school and we currently have two practice rooms which aren’t soundproof, meaning that noise escapes to the surrounding classrooms. As part of my development budget I’m keen to put in a bid for money to get the rooms sound-proofed, however I’ve got no idea as to how much to ask for!

The rooms are quite small (approx 4m x 3m max). The amount only needs to be a rough estimate at the moment but I do need to have a figure by the end of today.

Many thanks for your help