IMO, you've really got to judge how long it will take you to create a site like this—given all the factors the client requires—and decide on a fair hourly rate that makes it worth your while to do the work. Then charge on that basis (whether you charge by the hour or just set an overall price). It really doesn't matter what others might charge. If your price turns out to be a lot more than others', then it wasn't the right job for you. If your quote is a lot less than others', then you're in a good position to get the job.
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.
Don't forget you'll only get around 80% of what you charge. The rest goes in tax.
Not necessarily. The amount of tax will depend on whether the designer is an individual or a corporation, what country they are based in, and several other factors. Also, if you're referring to income tax or corporation tax, that will be assessed on the profit, not the amount charged to the client. In other words, it's what left after you deduct your costs, overheads and expenses.