If you need a new deck it is most cost effective to replace both at the same time. It is virtually the same amount of labor to replace both as it is to replace one, which I can typically do for $75 labor.
If your belt is worn out, chances are that you deck needs to be replaced as well. If you are not sure, here is how you can tell:
With your treadmills belt adjustment tool loosen the running belt tensioning bolts counterclockwise exactly ten turns on both sides. With that done, reach in under the belt from either side and visually inspect and feel the running deck surface. If it has grooves or feels rough it needs to be replaced.
Don’t forget to tighten your treadmills belt adjustment bolts exactly the same amount of turns clockwise putting the adjustment exactly where it wss before. Then turn on the treadmill to test for proper alignment.
Remember: Worn out belts and decks as well as lack of proper lubrication will put stress on your treadmills rollers, drive motor, motor control board and may cause premature failure in on or more of these components, in addition to less than optimum treadmill performance.
When in doubt order both. I generally recommend that you replace both at the same time.