Well, a fair number of people are switching from Paypal to Authorize.net, for whatever that's worth. Paypal just seems too focused on regular Paypal integration and doesn't really help much if you're using them as a merchant system. I've integrated SecurePay as well, but to be honest I wasn't very impressed by their mediocre API and total lack of documentation, so unless they've significantly improved in the last year or so, it's still behind Authorize.net. Did you have any other merchant systems in mind?
No, the Authorize.net gateway is not free, obviously.
http://www.authorize.net/solutions/merchantsolutions/pricing/
Your ongoing fees, unless you're using some of the advanced features, will be $20 / month + 10 cents per transaction + 25 cents per batch. Batches are run as frequently as you want them to be run - generally once every 1-7 days. If we assume for the sake of argument 100 transactions per month, with batches run every 3 days, that's $20 + $0.10 * 100 + $0.25 * 10 = $32.5 / 100 = $0.325 per transaction counting everything. Credit card fees still apply, of course, but you'll get those added on top with any system.
Yes, check processing.
No monthly minimum as far as I know.
