What a giant can of worms this is. Setting up a site can be both incredibly easy and a nightmare. Depending on the purpose of the page and its nature it can be as little as 6.95 a month - to hundreds per month to maintain.
I run www.vonconcepts.com (an ezine) for 6.95 a month but handle all the updates myself. For the same price I also run my book's site at www.servicewriterbook.com with simple PayPal links to collect my money.
The hard stuff is the shopping carts and encryption along with support forums. Setting them up can take a lot of time which translates into money. The software I use is from Macromedia and cost me about $1000.00 plus a month of my time to learn it.
To set up a site like my book's -- simple and fairly static, I would probably charge $300.00.... but it would be the client's responsibility to handle the hosting.
Kurt