There is no "ideal value". Stats are on a curve, and the value of one stat is contingent on how you have of the others, and vice versa. While there may be preferences in certain stat ranges, never take that as gospel. Individual setups can change things drastically, as does the fight in question. In particular, mastery is the top stat pretty much only on multi-target encounters; on pure single-target fights, haste and crit can often overtake it quite easily.
The general consensus is to try and gear for mastery + haste, as those tend to give the best increase overall, on average. However, that doesn't mean that it's ALWAYS optimal. Your own particular setup may very well favor something else, based on what level your gear is at, what trinkets you are using, and so on. Simcraft is a very useful tool for just that: not giving you a definitive guideline for what stat to stack, but for comparing concrete examples of gear you have or are considering to get. Do a comparison sim with the gear pieces available to you, and see what comes out best. In the end, that's probably a lot more useful than "mastery is worth X, haste is worth Y" because you need to work with what you actually HAVE. And there are no magical 955 pure Mastery pieces in every slot, or anything like that.
For most classes, there are certain diminishing returns at work when it comes to stats. Stacking one single stack is rarely optimal, due to the synergistic effects at work; the more you have of one stat, the more the value of other stats tends to increase, and so on. Where and when and how that happens, of course, can be tricky to determine. Simcraft is the best approximation.