I respectfully refer you to Skip's post above.
Good Lord! The idea is to provide weight, not soak up urine!
And 20 lbs of sand costs a lot less than $6.
I agree. 60lbs of course multipurpose sand is only $3.58 at Lowes. Seriously. Just buy the sand people. These sandbags are also double layered so it's not like the stuff is just going for fall out everywhere.