If I remember correct, I bought from Artfriend a piece of plastic-like board (architecture students will know the correct name) for $2.40 and it's big enough to make 2 pieces of bounce card. Sent the board to my friend's workshop and it was done in minutes. I got 2 pieces of super nice bounce card for $1.20 each. Of course, you still need spend a few dollars on velcro.
The motivation to make a bounce card for myself was because I find that normal bounce card is a little too small to my liking as doesn't spread the light wide enough. Hence, I made it slightly bigger than normal.