Give vangogh's coment a good read.
Something else to consider, you aren't being paid for all that time spent negotiating, getting requirements (asking the customer lots of questions), and coming up with the estimate.
This is a major reason that experienced developers/designers don't take small jobs. Often, more time can be spent in the administrative (read unpaid) tasks than in the actual development. Others will factor that into their estimate. This can make a small project look expensive.
Lets take this project apart a little (note, numbers are just made up)
2 hours - Discovering the customers requirements
4 hours - putting together a very basic design for an estimate
2 hours - looking for good images from your Corbis library
4 hours - putting together a good design/layout
15 hours - 1hr/page
4 hours - customer acceptance, system admin (getting it up on the customer's server, testing, etc...
Note, time can get eaten in a lot of other ways as well.
So, that's around 31 hours so far (notice that about half the time is actually spent doing the
work).
Now, it's up to you to decide how much your time is worth. $100 is about half of minimum wage. If you want a starting point, many (very) junior web developers start at around $24/hr.
Now, I understand that when you are just starting out that you are probably just trying to get any experience you can. But, be careful that you don't create a future expectation of working for very little.
If you aren't comfortable with asking for what really is a fair price, don't give a blanket estimate (ie. $500 for the site). Break down your estimate,
create a detailed list of tasks and the estimated amount of time each will take. Present this to the client. If they can see where the money is going, they are much less likely to have sticker shock.
If you take you car to a mechanic, they shouldn't give an estimate that says $500 to fix you car. They'll give you one that says $50/disks, $100/calipers, $150/labor, etc....
If the client wants to comparison shop, send them to me, I'll quote them $3k for the same thing (and I won't use iframes and tables

)