What the easiest and cheapest way to travel from Victoria coach station (london) to Barking train station?

How much will this cost?

On foot.

walk along to Victoria rail station and take the tube ... eastbound District line to Barking ... buy a single ticket ... cash fare £4 ... would be cheaper with an Oyster card but it's not worth buying an Oyster card if that's the only journey you're going to be making