If you have the MP Explorer card, the best option may be (depending on whether you have enough miles earned via your MP Explorer card) to buy the ticket for $421 and then use 42100 miles to get a statement credit for that amount through the choices program. Not the best redemption value, but you're using UA miles and still would get miles for your ticket. I would only use this at this rate if you're hurting for cash, flush with miles and need the miles earned on the ticket (e.g. for a mileage run).