Can you share what your fees are for authorize.net? They don't disclosing anything on their website...
Heres a ballpark of what you can expect with them:
Merchant Account ~ 2.4% per transaction + $.25 per transaction + $10/month
Payment Gateway $100 setup + ~$20 a month and.10 per transaction
You will need them for a Payment Gateway, but you may have a merchant account with someone else (for instance Paymentech)
I have had good experience with Auth.NET in the past and they seem to be a good value for medium transaction volume ecommerce sites.