Nothing; it is included with your regular subscription. The payment processor you choose will have its own transaction fees, and we support several outlined below.
To set up a new account, go to your control panel and Settings > Bank accounts > New account. Here are your choices:
Democracy Engine is a donation processor that integrates seamlessly with NationBuilder for any U.S.-based recipient (unfortunately it does not handle international recipients at this time). There are no set-up fees for Democracy Engine and the processing fee is a flat 3.75% plus $0.20/transaction. No merchant account is required and set-up usually takes a single business day. DE transfers funds to NationBuilder customers via ACH transfers. The donation cycle is Monday to Sunday, with transfers initiated on Wednesday and deposited on Thursday (more info here). For additional inquiries about Democracy Engine, you can reach them at email@example.com.