It appears the best deal works out to just about $0.03 per mile. This would have been good when combined with the redemption discount deal at the end of December last year, but I'm not sure it's such a good deal now unless you were going to buy miles anyway.

(Sorry all! I didn't see that the miles are posted after the fact, and not at the time of purchase. Thanks for the info - this might be a good deal after all!)