Recurly's Salesforce integration adds a field in the Subscription table called Monthly Recurring Amount. Where in Recurly's API is this value exposed for accounts or account subscriptions?
The Monthly Recurring Amount can be pulled from the subscription details (https://docs.recurly.com/api/subscriptions#lookup-subscription). The SFDC integration is pulling the unit_amount_in_cents, which is the regular subscription amount (not including any taxes, credits, coupons, etc).
It's important to keep in mind that MRR will rarely, if ever, equal actual revenue. That's because as subscriptions renew things like taxes, credits, and coupons will impact the invoices that's generated. Furthermore, when the subscription renews there's a chance payment will be declined.
MRR is a great value for trends but won't match actual revenue values.