So, we now have crafting rules, which are 5+CL of the item to be crafted.
And we've been having rules to calculate how much a given magic item is worth based on its abilities.
But we don't know about calculating CLs of custom items, at least from what I can tell in PFRPG.
Likewise, it's exceedingly unclear why the CL of one item that performs a similar function to another item can be drastically different. (Example, compare bracers of armor to ring of deflection. Scaling CL vs Flat CL.)
Of course, it's been said that CL and gold values of some items are an "art" rather than a "science."