I would suggest that you quote a price that you are happy to do the job for. If you think it will take you 3 days, then quote a price that you will be happy to do 3 days work for. Always focus on quality first and foremost, though!
My attitude was always to work for a wage that I was happy with, and if someone faster wants to undercut your price then let them. Nobody wants to be a busy fool. As your speed improves (and it will) you will find that your pricing will become more and more competitive.
Don't price yourself cheap as you may become despondent and start to sacrifice your quality, conversely, don't over estimate your own worth. It is difficult to earn as much as you think you should when you first start out but you should still be able to earn a fair wage.