So I take it you've set something like this up before? What would you guess is the real world cost of having a self hosted checkout?
I've setup literally 200 sites using Auth.net, on various canned and custom platforms.
Those who use paypal for niches like downloadable software and memberships get by just fine, but if you're selling something to be shipped, selling something to an Interwebs noob, or selling something B2B, you absolutely have to have a processer like Auth.net.
You'll find 2 modes, simple integration and advanced integration. SIM means that the user is redirected to auth.net for payment processing. AIM means payments are processed on your website, the user never leaves.
Keep in mind that with volume, comes more responsibility for security. PCI compliance comes into play in some industries.
My "gold standard" advice for companies is that if they average $10K+ in daily business, to take an extra step and install an application firewall and SSL terminator appliance. This protects you from a LOT of issues that come with XSS and credit card theft hacks.
Anywho, it's going to cost you about $50/month just to have the merchant accoutn, so make sure you can absorb that cost. It's also prettty nice to accept echecks and offline/phone/fax payments using auth.net.
