AmEx and Discover are set up for just the merchant account and charge a monthly fee, but once set up, can be interfaced with the MC/Visa accounts that are done with the terminal.
When I did mine, bulk of expense was up front for account set up; then monthly fees vary... as so the percentage transaction fees. Not necessarily "negotiable", but worth shopping for the least costly percentage and/or transaction fee. Cost of doing business these days.
Watch for total cost of business. The two biggest are, of course, the equipment rental and transaction fees. Some companies though, have "statement fees" that I've seen vary from eight to fifty bucks a month. Also get some kind of turnaround time for resolving of disputed transactions estimate, and make sure you know what ALL the fees and charges are before signing a contract.
I looked at all my options, and since most my sales are internet, I went with PayPal. No monthly fees. Higher per transaction charge, but my sales are not high enough to justify monthly. There are about siz zillion credit card processing companies. Go with a pretty big, well established one. This is your customers bank account data and credit card data going through. Hope this helps.