Two years ago, I purchased some bars of washing soap. The white more expensive one disappeared in a month. The yellow bar lasted well two years! I wash only my undergarments, lungis/mundus and the occasional blanket. Still, it is good value for money. If I had used detergents (made from petroleum), I would have gone through two dozen bars and it would have been more expensive too. So, the judgement is in favour of soaps on all counts.
IMPORTANT: Washing soaps need to be stored in an air-tight container. They seem to lose their softness if left out like detergents.