There are a slew of items that will be vastly more beneficial to you for that $100.00 than a sled. The $100.00 will be way better spent toward ammo, trigger time, optics upgrade, etc, etc, especially if you want to go long.
A rifle can be zeroed and loads worked up by using a good bi-pod and a rear bag, both of which you should use in the field for long range shooting anyway.