Chester is outside the MerseyTravel area, so the zonal system doesn't apply.
As Dzug states, the single fare is �4.30. A return fare is normally �5.75 but a Cheap Day Return will cost you only 5p more than a single ticket. (i.e. �4.35).
In Chester, just show your rail ticket on the bus to or from the city centre, and you'll get free travel.