This one is hard to determine so let's say you use around five tampons a day and your period last about six days. So that's about 30 a month. So if you buy a big multipack ($9.00) that may last you a month and a half Let's do the math:
$9.00 x 10 = $90.00 spent on tampons a year. As usual that doesn't really look like alot but...
if you buy 1 Diva Cup 
$17.00 to $20.00 x 1 = $17.00-$20.00 Diva Cup only needs to be replaced once a year.
You need to be pretty comfortable with your body to use this but it's worth the savings and keeping trash out of the landfill.
$90.00 - $17.00 = $73.00 or $90.00 - $20.00 = $50.00
