Firstly, you will probably need a Unique IP which costs $47.40 yearly or $3.95 monthly. You will also need a secure certificate which costs $189 yearly from Geotrust. Consider securing a better deal for secure certificate elsewhere.
Secondly, Authorize.net is a relatively good payment gateway. You should be able to find quite a number of competetively priced packages around easily.
Thirdly, you might wish to try credit card processing services from companies such as 2Checkout.com. With that, you will not need the Unique IP and secure certificate as such companies will handle all your payment on their secured servers thus reducing the cost.
Be sure to check out some of the promotional codes is offered in the forums if you plan to purchase Unique IP.
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