You're moving into a very crowded market and selling cheap websites doesn't mean that customers won't expect any changes, they'll still expect changes and this will eat into your profiit.
If you haven't done your market research you should do it before you start. Who's going to want this service, who are your competitors, what are they charging, what will you make from each website, how will you turn this into an income. When you're working out your costs look at the amount of time you spend gathering requirements, doing implementation, phone queries, changes required, after sales etc.
If you're selling a cheap or expensive website you'll still have to pay some marketing costs.
Sorry don't mean to be negative but there are thousands of web developers doing what you are proposing.