Registering for, collecting and redeeming mPLUS Rewards are all completely free!
The only times you'd ever have to pay anything is if you're buying the paid version of some of our apps, but you'd have to pay that to use the paid version of the app anyway - it's not a fee for using mPLUS Rewards. Also, sometimes we have contests where you can "purchase" additional chances to win (like a raffle) by trading in your points in exchange for entries, but no purchase is ever necessary and you can send in additional entries via snail mail for free, minus the cost of stamps and envelopes. :)
But, no, it never costs you anything to be an mPLUS Rewards member, collect points and redeem them for great prizes!