Elite status with 100% bonus on all flight miles makes a different value for miles.

For example, I receive 100% bonus with UA on all flight miles. When I am looking at a good trip just for miles I want my cost to be around 1 cent/mile earned. Therefore if I spend $300 I want to be getting 25,000 to 30,000 miles for the flights.

If you are not getting miles at close to 1 cent mile then you might as well buy a ticket if you are flying economy.

If what you want is a business or first class ticket then 2 cents/ mile could be worthwhile. A first class ticket to Europe is 100,000 miles and for $2,000 that would be a good deal or 120,000 for First to Asia/Australia and $2,400 would be a good deal.

For me personally I will generally only do a mileage run (taking a trip for no other reason than to accrue miles) if I am earning the miles at less than 1 cent/mile.

At that rate a business class ticket to Europe is earned for $800, First Class for $1,000.

There are really good promotions that allow cheap accumulation of miles if you plan well. The current AA and UA triple miles bonuses allow for earning at around 1 cent/mile if you have elite status and get 100% bonus flight miles already. For example, a San Francisco to Singapore roundtrip can bring in around 18,000 miles and even if you have no status (you will after that trip) you get triple miles =54,000 miles. The ticket can be purchased for about $540 right now if you have a 15% coupon (they are being traded on The Coupon Connection) and that is 1 cent/mile even for someone w/o status on UA.