The price I set on my soap bars is based on several factors including my labor, materials, and the local market rate. I keep a detailed spreadsheet on all my ingredients including shipping, labor, overhead, etc. so I know exactly how much per ounce one of my soap bars cost. Then I determine the price based on that and the local markets. Those can vary greatly from county to state to city, town, etc. For example, handmade artisan soap can bring in $9-15 per bar in NYC while out here in the Virginia country where I live no way can I sell a bar for that much.
For a 4oz bar I currently sell mine for $5. I also offer a discount for multiple bars.
Setting the right price for your soap is very important. I see so many beginners underselling themselves and others who haven't a clue as to how much it costs them to make a bar of soap. You should know everything about your costs and what the market will bear to set your price. Since I do a lot of local craft shows and farmers markets my price range is right where it should be to be competitive yet within the parameters of my costs.
Being careful how you shop for supplies is also extremely important at keeping costs down. When I first started it cost me almost twice as much to make a bar of soap than it does now. Learning how much to spend and where to get the best quality supplies will play a big factor in your costs.