The most common way that schools that cannot afford to bring David in is to share his time with another school in their area. You may partner with another school and cut his fee significantly. This also lets you create a relationship with the other school for the future. David can speak at one school in the morning and another in the afternoon. Middle schools will share his time with the high school in the district and vice versa.
Contact someone in the PTA/PTSA/PTO to see if they can contribute some of their funds.
Check with the school administration for funding from the Student Body Fund.
Contact you local Rotary Club, Kiwanis, Lions Club, Chamber of Commerce. You will have to make a brief presentation to their members for consideration of a donation from them. Most of these clubs are very generous with students.
Bring David in as your keynote speaker during any of the important theme weeks throughout the school year such as Red Ribbon Week, Respect Week, Drug and Alcohol Awareness Week, etc…There may be federal grant money available for programs like this in your school. Your school district or local government representative would be able to assist you with applying for this money.
Invite a local business that may support a youth motivational speaker at your school. Perhaps they could be represented some way at the school if their business is related to teens and or family.
Have a fundraiser with other students in your school.