I need to buy 51,000 miles for a trip I plan to take in December. Could you possibly direct me to a site that handles this kind of dealing?
Purchasing miles only makes economic sense when buying small quantities because the cost to buy miles is higher than their value when redeemed for an award ticket. My general advice is to purchase only enough miles to reach an award threshold.
The airlines reinforce this notion with their policies regarding the sale of miles, which restrict the number of miles program members may purchase per year. While the airlines won't admit it in so many words, implicit in their restrictions is a warning that buying miles in significant quantities isn't a very good deal.
For example, you can buy 25,000 United miles—enough for a round-trip domestic coach award ticket—for $707. Since most round-trip domestic coach tickets can be purchased for far less than $707, it just doesn't make sense to pay that much for miles. Plus, it's often difficult to find an available award seat for 25,000 miles, whereas you can always spend actual money for a ticket for a flight that's not sold out.
Miles can be purchased from two sources, either from the airlines or from mileage resellers such as Points.com. The pricing and annual caps are mostly the same, so there's no significant advantage to doing business with one over the other.
Among the largest programs, you can purchase as many as 40,000 American miles per year for $1,000 plus a processing fee and applicable taxes, or as many as 40,000 United miles for $1,110 including a processing fee and taxes. Delta limits SkyMiles members to 30,000 miles per year (for $887), and Northwest WorldPerks members can buy up to 15,000 miles annually ($445). Other airlines have their own prices and mileage caps.
Before committing to a mileage purchase, carefully evaluate the value proposition. Small lots, while expensive on a per-mile basis, can sometimes be justified when topping off an account. If you're considering purchasing more than 10,000 miles, you're probably better served by buying a ticket instead.