Hi there,
For volunteer jobs, you may provide the following benefits:
• Pre-paid Petrol Cards
• Lunch & Dinner during Volunteer Work
• Pre-Paid Phone Cards if they need to use their mobiles
• Some petty cash as a bonus
• If they have to achieve certain targets, why don't you give some bonus for certain achievements? For example, those who achieve the minimum target don't get anything except what is mentioned above; those who achieve an average target get some cash or vouchers, and those who achieve the maximum target get double what they earn as salary (if any) during their work.
Even though it's a volunteer job, you need to make it as interesting and fun as possible to keep them motivated. Remember, they have invested their time to help you, and time is money.