This is a tough question for most people who are just getting into "the ebusiness" or have been in a while, but getting more serious work.
Do some research. What are your skills, and how do they match up with other small or large companies. I do A LOT of backdoor marketing he he... It's fun acting like a customer, but the information gathering is great! Ask other developers the same request your client is asking... Fast way to sort out how much to charge.
How much do you charge for on going business.. or to keep your business running? Are you doing this for a living or part time?
If you are doing this work for a living. Then you want to figure out what your monthly income needs are to survive. Medical, Auto, Loans, Utilities, blah blah.. Most forgot item is - play money, and money left over to build a savings for your company. What will be your work load?
After you have all your numbers down - there in the bottom line - you will see what you should be earning.
I will give this though. If you are good and produce quality work - charge what your worth. Many companies that come to look to get online have already planned for the investment and usually come to play! If your clients are personal websites - ummm you know the rest of the story.
What you are doing is running a business. Like every business you need a plan of attack. You also need to survive to keep your attack full on. Selling yourself short is a loss of money.
BEST OF ALL!
If you don't know how to do your research, build a business plan, or set up your accounting. I suggest signing up for a small business class at your Community College. You may also call your local chamber of commerce and and they will be able to put you in touch with some workshops or classes to get you rolling. Chamber of Commerce is also a great tool to drive clients to your business, and NETWORKING ;)
Best of luck and hope I was some help.