Karin:
The PayPal option is great for taking credit cards over the Internet.
As a benchmark for taking credit cards at shows, etc. I have a friend who has his credit card service with a straight knuckle buster swipe machine, and he pays $8.50 per month and I think he paid about $30 to get setup. There are a lot of deals out there for taking credit cards that really hit the vendor with big upfront fees and expensive monthly service charges. Look for one that comes reasonably close to his and you should do just fine.
BTW: You can get the more expensive dial and transmit swipe machine that hooks to a cell phone too. Those are not always reliable as they are dependent on always having electricity at your booth and a good connection on your phone. They can be handy because you get the authorization code immediately rather than having to process it through your CC service provider. Be sure you get a standard knuckle buster too so you are not caught in a bad cell area with no way to take cards.