$5/mo to $40/mo depending on what level of service you want. You can get cheaper and you can get more expensive, but neither is likely to be a good idea for a new business.
Most people with average needs should be paying USD $8 - USD $20 per month. $8 if you want it as cheap as possible, $10 for a good mid-level of service, $20 if you want some extras.
If you want your own secure site (and the fixed ip that requires), good cart software provided by the host, etc. expect to be paying $40/month plus secure site registration fees (can't remember what those are - $200-$250/year rings a bell, but don't quote me on that).