Domain values are based on a combo of supply and demand as well as quality of extension. .Com is the obvious king. With all the myriad cctld's out there, registering even a very rare name is going to be very close to reg fee. esp if the cost to register it is high. A suprising sale I saw on Sedo today was arab.cc for $850. That's very high for that ext.
I don't see how it would much of an investment. A good rule of thumb is that if all the domains you are looking up are still available, they're not as good as you think. That's not to say you can't find some gems but with all the cc tlds out there, the supply keeps the demand low.