I would have to say the Oneworld promo of 1999 is my favorite mileage earning steal. My neighbor and I did the necessary 20 segements to get the 4 X 100,000 mile bonus on AA, BA, CP, and CX. Total cost for each of us was $1,750.

The routing was:

STL-ORD-LHR-MAD-PMI-MAD-MUC-HEL-TMP-HEL-MUC-MAD-LHR-ORD-STL-ORD-YYZ-YUL-LGA-YUL-YOW-ORD-STL

actual miles: 17,308
bonus miles: 400,000
Total miles: 417,308
per mile = $.0042

Beyond the incredible value we received for our miles (I took Concorde this year using my BA miles), we had a great time. He's 75 years old and I worried at first that he wouldn't be able to keep up or that it was too much money for someone on a fixed pension, but I think he enjoyed it more than me. Quite a challenge.