I think all WaveNet speech is still being generated server side instead of on the client hw? So there is a cost to associated to it.

If Google Duplex is a paid product, maybe it just enables running WaveNet on Google Cloud with larger models and higher quality settings .

Speech produced for Assistant doesn't make any money so the server side cost has to be minimised.

One day we'll have all this client side, on specialised ML chips on devices.