I'd go with the X/per oz pricing.
If you go with the formula of cost x 2 or 4 or whatever - then your pricing will vary a lot. If you are just starting out, you are probably not buying in the really large bulk amounts to get your pricing down. So your cost per ounce will be a lot higher. Yes, you need to recoup that, but unfortunately, the market will not necessarily feel it is their responsibility to take care of that for you. They will only pay so much for a certain size bar...so if you want it to sell, then your pricing has to be in line with what they are used to seeing.
That being said...Here is a pricing strategy that I use a lot. It is simple marketing psychology, but it works: I have the 2 oz "guest size" bars. Conventional wisdom says $2 a bar, right? I price them at $4 each regular, then have a sign saying "show special 3/$10" - and they all sell like crazy.
It's that "2 for" or "3 for" pricing that they really love. Also - I let them mix and match within the size category.
I am new to soap as well, but I've run a beeswax candle biz for years, and I have found that what is true in candles is the same for soap/lotions:
Consumers want 2 things:
1. the feeling that they are getting a deal
2. Choices (mix/match, custom gift bag, etc)
just ny 2 cents