Yeah - if you say 'purchase items' then its not basic...
This will involve some kind of database by the sound of it, and as yoshi said, you will have deal with some kind of validation of details. Your server will also NEED SSL if you are taking credit cards, which costs a bit each year to get. (About $100 to set up initially and then about $250 per year after that)
But i wouldnt buy anything from a site if they didn't advertise that it was secure.
What exactly are you trying to sell?
I have never done a db site for a client, but these fetch thousands plus more...
If it doesnt involve databases, and you can get around having to validate details, then forms makes no difference to the design.
A basic guide for how much a site 'could' cost would be say: (and this is for the site only, not the extras, and it is just an example)
You could charge $200-250 for the front page.
You could then charge $50 for every additional page.
Charge extra for images/flash
Charge even more for php/perl/etc
Then you still need to pay for domain/hosting.
You also need to get your site onto google/yahoo/msn searches. You used to try to get your site onto 'every' search engine, but now 90% of people use these three, so there's no point.
The best way to get onto google is to get someone who gets spidered regularly to link you.
Yahoo my site just turned up on. I think they steal from google now.
You will eventually turn up on msn as well.
