Choose an hourly rate that you would be willing to work for. If you have a job then i'de choose a rate that is slightly above what you get paid in your job (to start with). Then decide how many hours it will take and work it out. As you get more work and your productivity increases, increase your rate. Someone whos on £50 an hour can probably get 3 times as much done in an hour as someone whos on £10 an hour. (OR 3 TIMES THE QUALITY)
Don't quote too low. It can be easy to feel like you are quoting too high but if thats how long its going to take you then that is the price you should quote. HOWEVER if you are trying to build up a portfolio and are looking to get future work then maybe you could cut down the price in your own interest, but not in theirs.
In the end I think the price will determine the end quality of the website. Prices for an E-commerce website can range from a few hundred to thousands. The more expensive one probably looks a bit nicer and has a lot more attention to SEO whereas the cheap one is probably just a quick opensource job. I'm saying this because you also need to decide the quality you want to offer.