The cheapest way would be to cut some signs out of plywood. Another thought is this:
I use to make my own signs for my business to put on the side of the road. The material that they use for voting signs, you see on the side of the road at election time, is called "coreaplast". There is a place in Houston, just east of the Astros Stadium called "Reese Supply Co." They sell a 4ft X 8ft sheet of coreaplast for about $8. If you cut that up into small signs, you might beable to get 15 signs out of one sheet, depending on the size of the sign you want to make. I use to be about to get 8 signs out of one sheet.
You can get stick on vinyl numbers and letters in a pack, at Office Depot for about $3 , depending on the size that you want. If you go this route, for 18 signs, you might be looking at about $5 a sign. Pm me if you need some more advice, or call me. I believe you have my number since we have played together in the past.