Rough figures; an IAE V2500 engine on the A320 burns about 7kg of fuel per minute at idle thrust so single engine taxi-in is worth it. We (UK airline) are also allowed to do a single engine taxi out as long as several conditions are met. I recently did a single engine taxi out on a 321 and it got rolling with one engine at idle thrust at about 74 tonnes. If the gross weight is above 75.5t we have to start both.