I recently took a trip to South America, A $653 fare yielded 16,221 OnePass miles including elite and online booking bonus. (cost per mile 4¢)
15,000 OnePass miles transfered to Amtrak Guest Rewards = 15,000 points.
15,000 GR points were redeemed for an Amtrak one zone standard sleeper reward for 2 that priced at $888. (yield per mile 5.92¢)