Metro riders may want to consider waiting to buy a SmarTrip card, even though the transit agency plans to soon charge higher fares to rail riders who don't use them.

The agency is planning to drop the price of the cards from $5 to $2.50 to encourage riders to use the reusable plastic cards. But the change isn't slated to occur until Aug. 29, according to a Metro report.

Consider this the latest confusing fare change at Metro. The transit agency boosted fares on June 27 but plans to increase other aspects of fares -- including charging 25 cents extra per rail trip for users of paper fare cards -- starting Aug. 1. But it won't reduce the cost of the SmarTrip cards until just before Labor Day. Other SmarTrip card changes are expected this fall.

So riders may want to do some calculations as to what makes the most sense: buy the plastic fare cards before Aug. 1 if planning to take more than 10 rail trips before the end of the month? Or wait until the price drops?

Bus riders already pay a 20-cent differential when they pay cash instead of using a SmarTrip card -- and they lose out on the transfer discount when switching from bus to bus or train to bus if they pay cash. So buying a SmarTrip card now makes sense for frequent riders.

Metro decided to reduce the cost of the plastic fare cards late last month as a last-minute gift to riders burdened by the new fare increases. But the transit agency says in a report that it needs the extra time to slash the cost because the outlets that sell them, such as CVS and Giant Food stores, must change the signs and receive updated card packages.

Some transit agencies, such as Boston's MBTA, give away such plastic cards for free. But the SmarTrip cards cost Metro $3.40 each, according to the report. Once the cost to riders drops to $2.50, the agency will be losing 90 cents on each sale.

Metro has about $4 million in reserves from the profit it has made off the $5 cost of each SmarTrip card. It expects to draw that down to $2 million by June 2012, the report said.

The contract for the current vendor ends in 2012, said spokeswoman Angela Gates, so it's not clear how much the cards will cost the agency in the future. But they could become another subsidized cost for Metro to carry, unless it finds a cheaper way of offering them or an alternative payment system.