These will vary from bank to bank and from locale to locale, but -- $40 - $80 per month rental, merchant fees between 1.5 and 4.5% of each transaction depending on your sales volume, average ticket size and credit/risk worthiness.
If you do not already have a registered business and an established bank account it is going to be very difficult to get a merchant account - you will have to go through a third party like PayPal or another payment clearing house, which will cost you several extra %. In some cases, a new business doing online sales will be asked to put money - sometimes $10's of thousands - on deposit as security against chargebacks and fraud.
If you have an established business, check into your locak Better Business Bureau or Chamber of Commerce because they often have preferred rate offers from banks for their members.
And remember, as the merchant you are liable for chargebacks from claims by the customer, or from fraudulent tendering of the customers' cards, for six months. The bank WILL take the money directly out of your account, and you have to fight to prove that your charge was legit and the goods were delivered. A signature on delivery is essential for every single transaction.