If bags are too expensive, you could prepare small packets for participants. Buy a pack of brown paper or white paper breakfast bags, which should be very, very cheap and put a handful of candies and stickers from sponsors inside. You could also put other small things: confetti (you could make some on your own with a small punch hole cutter!), a handwritten notes with “Thank you for coming!” or anything else you could think of. Tie with a ribbon or use sticker to close the bag. It will look lovely!