How do you Value Models as an Investment?

In my opinion, I value a model's rate based on many factors, one the uniqueness of their overall look, if its easy or not to just replace them by another model with cheaper rates, the more unique their look is locally the less competitive that model will be meaning he/she can rate themselves pretty much any price if their look is highly demanded (supply and demand mentality). 

Another factor, is how many national/international print publications they've been published into and how recent? To me, the more recent the more value they hold because their name/brand has been recently exposed to a pretty large audience and (again) if recent that exposure can be rubbed onto your brand as well which can be worth something or not to you if you want to gain that type of exposure which of course depends on your target audience and the model's publication they've been published into audience as well. And the more national/international publications the more known and bigger following that model has or dont have which depends on the model if he/she is at all branded in any way, or not, many wealthy models are unknown to most of the world intentionally (from my observance).

Those are 2-3 pretty big major factors that come into play when I see a rate when I'm given one, if its a valuable investment to invest into that model and get as much of a value for my dollar spent. It may or may not be fair, but to me I try to be strategic as  much as possible with investments and spending, its not just about on equipment or other business costs, its also investing into paying some models and the right ones for the biggest return on that investment. 

For myself I don't pay any models without knowing I have some chance I can sell posters or some other kind of prints from those photos and GENERALLY, they tend to be bikini, nude, or implieds from female models, or male models shirtless. If its not any of those its extremely rare to make a print sale from clothed modeling other than for stock and advertising but that's entirely different world.

Others may never need to pay any models, depends on the type of photographers. I want to have a small or medium line of poster sales so I need to pay models, as well as one of my goals is to get published into men's magazines like Sports Illustrated, Maxim, etc... So this can give me the best opportunity by working these types of shoots.