I'd recommend talking to a few guys in the industry like Art. Get a quote for dial up based processing, online processing, cell processing, etc. Make a table for the equipment costs, monthly costs, additional infrastructure costs, etc - the answer should become clear.
My gut feeling is a traditional swipe unit that dials up would be most cost effective as you'll get a good rate and low equipment cost. You'd need a phone line (we can deliver those to most of renton for ~$20/mo) but that would be less overhead cost than internet or cell based processing.
