Most of the business I've worked for that accept credit cards pay anywhere between 3 and 6 percent of the total sales transaction in fees. The higher the volume, the lower the percentage - but if you don't do much in credit card sales you might even pay a 'per transaction' fee too. Then on top of that you have monthly equipment rental charges for the physical credit card machines and a software charge if you elect to poll the information to your pc instead of just sending it to the credit card processor from the credit card machines. The major reason a lot of business don't take american express or diners club is that their fees are higher than visa and mastercard. Also, the amount of time you allow the processor to send your $$$ to the bank once the items have been processed affects your rate.
The tradeoff for all these fees/charges are twofold - happy customers that don't have to carry cash and increased business/cash flow for the seller because people that don't have the $$$ in the bank can still purchase your goods. Seems to me that it's a pretty reasonable process all around.