The price of a shopping cart site depends on a lot of factors - how many products you have, how you handle shipping and taxes, how complicated a design you want, etc. 

As a good example, see http://www.antiqueables.com This type of javascript shopping site would be priced as follows: 

[1] Main Web Site Design: $120 / 3 pages 

For example: Welcome, About Us, Frequently Asked Questions. This would include a professionally designed interface including custom graphics and "look and feel", as you can see at the Antiqueables site.

[2] Shopping Cart setup: $95 

This would include 15 items on 3 product pages, plus checkout pages

[3] Additional Product Pages: $25 each 

Additional Product Pages include 5 products/page. Scanning and Web optimization of your product photos is included at no extra charge.

[4] SSL Setup: $120, $15/month 

SSL stands for Secure Sockets Layer, and would allow you to accept credit cards on your site securely (see SSL info)

[5] Monthly Hosting: $14.95/month for 10 MB of server space (www.yourstore.com) 

Sites are hosted on "industrial strength" web servers on redundant multiple DS3 connections to diverse internet backbones. 1 DS3 = 28 T1 lines. In English, this means your pages will load quickly, and your site will be online 99.5% of the time - guaranteed

[6] Domain Name Setup: $29.50 one time 

Domain Name Setup: the cost of setting up DNS service for your domain name.

2) What other costs are involved?

You could have other costs also, (or you may have these things in place already) including your own merchant account and software for processing your customer's charges, your domain registration fee with Internic ($70 for 2 years), and your digital ID/certificate (see SSL info)

From there, you can maintain the site yourself, or pay us to do it on an as-needed basis ($60/hour). If you would like to "do it yourself", and you don't know javascript and html, it would be best for you to invest in a more expensive "user friendly" shopping cart system, which will add an additional $500 - 2,000 in software costs. (for an example of this type of software product, see http://shopsite.com/products/manager.html) 

3) Where can I find out more about E-Commerce? 

Real World E-Commerce

Never obstruct someone from trying to spend money

Z-Commerce Daily

An overwhelmingly complete list of resources from PC Webopedia