[Jamesk]

The simple answer is to amend the rules to limit the distance the motor is allowed to be from the axel line (ie where the driveshafts and diff are).
You would not affect the postion of the motor ie mid or rear engined but would stop extreme front mid engined 2wd's.

The only problem with this is it limits innovation, but innovation does increase costs, not a simple question if you look at it in a bit of detail. Could maybe have a differant weight or limint tyre sizes, but this usually added cost too.

jK