Following are the limits I use -
Domestic - $250
Europe - $400
Japan - $550 ( Mileage Runs )
SIN/HKG - $650 ( Mileage Runs )
If the price of a ticket is above these numbers, I use an award ticket.
Then again, it depends on the airline too.
I burn Dividend miles as fast as I earn them ( who would'nt at 4x). Using AA miles is a taboo unless they are being used for an international itinerary.