I think 2 cents a mile is a good measure. So if you use them for a tix for say around $500 then you are doing well but as the previous person mentioned, the best usuage for most frequent flyers is thru upgrades.... Of course don't forget that buying a tix earns you miles so the real cost of the $376 tix maybe closer to $276 (about 5K miles if you are an elite and fly RT from DFW to LAX).
Just my 2 cents (a mile)... ^