They cost a few hundred bucks. District judge out in California said their mag ban without grandfathering violates the Takings Clause so no doubt this does as well. They'd have to prove, in court, that bump stocks endanger the public to the degree where they may be seized without recompense. They'd have to argue an AR/AK outfitted with one is identical to a machinegun, even though (statutorily and mechanically) it isn't. Will be interesting to see because someone will sue.
Another problem is the definition of a "machinegun" (mechanical function) in the GCA makes this a problematic rule. Bump stock doesn't increase the cyclical rate of fire. The gun does not fire with one pull of the trigger. One can bump fire an AR using a belt loop. They gonna ban any clothing that requires a belt next? Come on.