I made videos on how to make your own monofin:
It cost$20 to make your own rubber one:
http://www.youtube.com/watch?v=YbaDYmGUhbA&feature=plcp
And $5 for a ducttape one!
http://www.youtube.com/watch?v=nld1wforX_8&feature=plcp
I hope these videos help!