The technologies involved are one thing, but it's really the scale of the site that dictates the price. 'They' would charge depending on the size of your catalogue, since this would determine data structure requirements and hardware requirements. Linking to secure payment systems on the banking networks isn't cheap. With regards to where you start... are you looking give someone a quote, or get a quote for your own site?

Building a basic web site is very easy, you don't even need special software (you can compile html using notepad). Nicer websites use more expensive software (frontpage/dreamweaver) and to add more pazzazz you can use Macromedia's Flash/ Fireworks/Shockwave etc etc. Get yourself a cheap html book from Amazon. If you want to host a large database you will require something like Microsoft's SQL server or Oracle's equivalent and to add the SSL functionality requires even more technology/cash. Just don't use the host that AB uses if you don't want to upset your customers.