A few bucks a month for a standard site would be what you're looking at. If you're looking at running an online store such as Magento or other shopping cart, then you'll want to find a host that supports the software, and again you can probably pay as little as $5/month.
If you're processing credit cards through a 3rd party service (paypal, 2checkout, worldpay) then you're all set. If you're processing credit cards yourself through authorize.net for example, then you'll also want to have an SSL Certificate for your site.
SSL's are their own separate charge, and will cost more for hosting since you need a dedicated IP number to assign the SSL to.
Prices on this are higher, but just as random with hosts
For us, we're $20+/month for dedicated IP hosting packages, but I'm sure you'll find cheaper out there.