Simple cost calculator/projector for Subscription payment services (Google Docs)

I'd love any feedback possible on this thing!

In trying to figure out which subscription processor to use on our new web app, I wanted some way to see how our credit card/subscription processing costs would scale with more subscriptions.

It's currently setup for 1-500 subscribers, but I'm happy to add more if requested.

Hopefully people find this useful! There are probably mistakes and I'll fix any that are reported in this thread or e-mailed to me at bradley@getmochi.com.

(p.s. right now it only checks Braintree/Paypal as gateways and Spreedly/Chargify as billing services)