I purchased all these items (except the knife) from Wal-Mart for $12.77, which made
12 balls, for a value of $1.06 per ball (at the time of writing.) Most of these items can probably be found around the house, so you may be able to make your juggling balls for free.
Steps
Cut a small slit (3-4 centimeters) in a tennis ball. (Note: Be careful here, the knife could easily slip.)
Fill the tennis ball with your chosen filler until you can hold it up to your ear and shake it so that it makes little to no noise.
Cover the slit with a single strip of tape, be sure none of the filler is going to fall out.
Cover the rest of the ball with the tape, add a pattern if you like.
Repeat steps 1-4 until you have enough balls to juggle.