I've bought an XM MyFi two months ago, they are regularly $300, but I bought an unused one missing it's manual for $250 at Best Buy. It is a great product for baseball junkies, as you can listen to Dibble and Kennedy during the day, the Ripkens, Holden Kushner, Mark Gray, Charlie Steiner,etc..
I comes with a plethora of accessories, but mainly a base battery charger stand, an antenna with cord, and I use a short wave radio I've had forever as it's speaker(it's signal is even strong enough to work on other radios in the house)..
In between being hooked to power in the home or car, it will slowly lose it's charge, and I've yet to test it's duration, but it it seems that it would stay charged for hours longer than I have ever been between charges.
I'm not sure of Walmart or not(I've never seen it there), but Best Buy,etc..carry it, and it is well worth it!