Recurly_PlanModel.php

namespace Craft;

class Recurly_PlanModel extends BaseModel
{
    protected function defineAttributes() {
        return array(
            'sortOrder'     => AttributeType::SortOrder,
            'code'          => AttributeType::String,
            'max'           => AttributeType::Number,
            'min'           => AttributeType::Number,
            'baseEndpoints' => AttributeType::Number,
            'endpointCost'  => AttributeType::String,
            'annual'        => array('type' => AttributeType::Bool, 'default' => false),
            'baseCost'      => array('type' => AttributeType::Number, 'decimals' => 2),
            'endpointCost'  => array('type' => AttributeType::Number, 'decimals' => 2),
        );
    }

    protected function getAnnualSavings() {
        return $this->annual ? $this->baseCost + $endpointCost * $baseEndpoints : 0;
    }
}

RecurlyController.php

    public function actionGetPlans()
    {
        $plans = craft()->recurly->getPlans();
        $this->returnJson($plans);
    }

RecurlyService.php

    public function getPlans()
    {
        $records = Recurly_PlanRecord::model()->findAll();
        $models  = Recurly_PlanModel::PopulateModels($records);
        return $models;
    }

So with this, I can do $model->annualSavings to get exactly what I want, but since it isn't defined as attribute, I don't get it in $this->returnJson($plans).