At Walgreens this month they are offering a $1 rebate (can do up to 10 times) on these PlugIns. Regular price for these are $1.99 and you can find a BOGO coupon (S 9/14-36 and here). Here's my math:
Buy 2 @ $1.99 = $3.98
Use 1 BOGO = ($1.99)
Submit for rebates online = ($2.20)
Result = ($.21)
Note: When you submit for a Walgreens Rebate online and have them put it on a Gift Card they give you an extra 10%. Great way to cover your tax cost and interest for letting them borrow your money for the month.