From what I understand, a soap can weigh more than the amount listed on the label, but it cannot weigh less. If it weighs less then you are misrepresenting your product to the customer because they aren't getting as much as they think they're getting.
The way I price it is I take the cost of the batch and divide it by the number of bars it produces. If each bar is slightly different and some people pay more per ounce and others pay less, oh well. As long as the weight on the label says less than the actual weight you're not misrepresenting your product and people can make their own decisions about which bar they want to buy.
For example, if in a certain batch the bars range between 4 oz and 4.5 oz I'll put 4 oz. on the label. Those who end up with a larger bar are just lucky. I price them all the same. I take the materials cost + overhead + profit to get my wholesale price, and then multiply that by 2 to get my retail price.
There's a great article on Teach Soap about pricing melt and pour:
http://teachsoap.com/pricing-your-melt-and-pour-soaps/I also found this Etsy lab extremely helpful:
http://www.etsy.com/blog/en/2012/etsy-s ... -workshop/This blog article about common soap labeling mistakes is really good too:
http://blog.mariegale.com/five-most-com ... ng-errors/