Considering the Warp Stone is available to ALL Demigods (currently) before Erebus could dream of getting Bat Swarm, this is not really a balance issue. The Warpstone has a higher range than the first level of Bat Swarm.
Even if the Warpstone is adjusted the way I suggested (20 yd Range, 5000 cost) it's still available AROUND the same time as the first level of Erebus bat swarm in a standard game, and does NOT cost mana to use.