I run 5 websites (none in retail though). I can say that this is a hard question to answer. Clearly, overstock.com will cost more to run than joesglass.com. I personally use bluehost.com for hosting, so my monthly expense is under $10 for all 5 sites (non-retail though…). I see anywhere from 8k to 20k visitors monthly with no speed issues. You should probably look into some e-commerece software solutions like this:

http://smallbusiness.yahoo.com/ecommerce/

It looks like their starter package is about $40/month, and its scalable in case your business grows unexpectedly.

When considering a business online, the most important factors for you are going to be up time and customer service. I have run 5 websites for the last 6 years and I have never failed using HostGator. They are very reliable and provide really good customer service. The price is hard to beat also. For what you want to do, you are looking at about 10 bucks a month with hostgator.

Most people underestimate the level of investment that an online business requires. SEO/SEM will require both expertise and money, also being able to know how to structure your site and ensuring the backend systems are good will be critical. Without knowing more about the type of online business you want to start, it will be difficult to give you a good answer. Please consider that the two extremes of online businesses are a) a business where you already have the customers but need to facilitate their further purchases via an online channel, and b) attracting customers who may want to purchase your products if they are presented to them. These two models (and all varieties in between) are so different.