For off-the-shelf models, i.e., soaps I've already made, the cost is usually $5-$6/bar, or $70-$85 for a batch of 16 bars. You save about 11-12% by buying a whole batch. Additionally, I make custom batches, and the cost depends on the ingredients.
My "standard blend" of oils is a mixture of olive, palm, coconut, and castor oils in a ratio of 3:4:4:1 (that is, 25%, 33.3%, 33.3%, and 8.3%), and has a base price of $55/batch. That is, if you get an unscented batch of soap, it will cost $55 plus shipping.
If you want fragrances or other additives, add their cost to the $55. I generally aim for $15-$30 worth of additives. For Gingersnaps and Uber Lavender, I use $15 of fragrance. For Abeda Deluxe and Chillax, I use $30 of fragrance.
I have a lot. Working on getting a list together. In the meantime, ask for what you want. If it's listed as an ingredient in one of my fragrance recipes, I ought to have it. Be aware that many popular floral essences are super expensive. Rose and neroli (orange blossom) are among the most cost-prohibitive, so I usually use those in blends with cheaper materials. But if you want to put $100 worth of neroli essential oil in your batch of soap, that's up to you.
Currently I am using the US Postal Service. Their flat-rate boxes are an economical choice. For 1-4 bars, I use the small flat rate box (postage = $5.35, so I charge $5.50). For 4-32 bars, I use the medium flat rate box (postage = $11.35, so I charge $12). For 32-64 bars, I use two medium flat rate boxes ($24 total charge).
Contact: open.source.soap at gmail