The least expensive way is to a WDW Transportation bus from POR to Downtown Disney (second DTD stop) and then to take the Lynx Link 111 bus from the Downtown Disney West Side Transfer Center to Florida Mall.
Here's the schedule:
http://www.golynx.com/assets/userfiles/routemaps/111.pdf
The map on the schedule shows that part of the route is even on the I-4 and SR-528 freeways. I've never taken a Lynx bus anywhere, so I can't share personal experience.
The route operates every half hour and requires no transfer. It seems like a bargain for just $2.00 full fare.
The easiest ways would be to use a rental car if you have one or a taxi cab or towncar service if you don't. But the cost for a cab or towncar will be much, much higher.