Still not sure why any of this would add cost. it all happens on the iphone which already has these sensors. If you can tell in which direction the iphone is moving thanks to gyro/magno/accelo AND you have a distance from BT device of reasonable resolution, you should be able to figure out the location of the device you are tracking with small movements of the iphone.

I'm not great at geometry but I figure it should be possible. Not sure about accuracy though.