Each person has a different definition of what a good deal actually is.

For revenue tickets (where you're paying cash and earning miles), most people calculate cents per elite qualification mile, while others use cents per redeemable (award) mile. Elite qualification miles (EQMs) cannot be used for awards, but they do count towards elite status. Personally, I think a good deal is under 5 cents per EQM, but others are more ambitious and looks for fares under 3 cents per EQM or even lower.

For award tickets, you can find the price of the same revenue ticket and then count how many cents you are "earning" per mile that you spend on the award (or hotel night). Let's say you spend 25K miles on a plane ticket IAH-ANC that normally runs $1000, so you are getting 4 cents per redeemable mile. You will also have to pay the taxes even when using miles, so watch out for those in your calculations. The best redemptions tend to be on first class tickets though. It is important to note that some people dislike this calculation because they would never buy the cash ticket in the first place and are only doing so because they can get it for "free" using miles.