Personally, I value Skymiles at about 1.2 cents per mile. So a ticket needs to cost more than $300 to use miles instead of cash. Don't forget to factor in the miles you won't be earning when you fly (free tickets earn no miles). If you have status (bonus miles) or are approaching a different level, or if the trip is across country instead of across the state, those earned miles mean more.
By the way, I am new to this board, but I have been flying Delta since '99 - I have been Platinum 6 years and Gold once, Silver once plus had status on several other airlines. I'm not trying to pat myself on the back, just saying that even though this is my first post, I'm not as green as it may seem.
I plan on watching the Delta and American message boards mainly. Any other boards that are particularly good and general? I guess I got off subject. oops.