I would not use authorize until you have enough members to justify the cost. On average you'll be looking at spending $250+ in set up fees, plus the fees for each charge. That price will vary with each distributor, so if you do go through authorize, make sure to get quotes from a number of different distributors/partners.

You might also want to check out this link for an alternative:

http://www.adultwebmasters.org/Making_M ... t_Systems/

I haven't looked at any of the files for the payment system. I had just assumed that there's an API for paypal in the template. If so, it shouldn't be too difficult to swap this out with the API for any other processor.