Proof coins vary in value because of their availability. Some years are more available than others.
And proof coins also vary in price because of their quality.
The term proof defines the process by which coins are minted and it most certainly does not guarantee that a coin will be worth owning.
While it is true that proof coins do have smooth fields, and in most cases the fields are reflective. Proof coins also have a highly detailed design and they do have well defined, glossy edges.
But (and this is the important bit) if they have been mishandled over the years and toned harshly or perhaps accidentally been used, they are not, in our book worth having. They are souvenirs only.
For you as the buyer that means that there are poor proof coins, average proof coins and proofs that you simply cannot take your eyes off because they are so sensational. And the sensational proofs are extremely rare and will attract a substantial price premium over one that is poorly preserved.