Garmin Speed & Cadence Sensors

I use a Garmin 920xt to map my Mountain bike rides, but because of the trees blocking out signals my milage accuracy was almost 20 % off { less } I was told a speed sensor would help, and it has , this sensor compared to the magnet sensor that has to line up precisely to the receiver,are prone to getting knock off line and dificult to mount on mountain bike frames and forks , this design is great as long as those bands hold.The cadence for around 20 bucks was a why not,Now if they can make a power meter for that price I'd be set, there is no way a power meter shouldn't be under a 100.00 bucks and probably cheaper than that