What do you think the low poly count per model is nowadays ...

skyrim has 15k
gears of war has 30k

If you want to go for higher end developers shoot if that. IF you want to appeal to the indendent developers who get stuck with unity and other stuff, I'd shoot for 10k max, someone correct me if I'm wrong.

Agree with you Michael

The reason we have lots of different 3D model types (from minimal detail to very high detail) is because we have lots of different hardware platforms.

So if you are producing content you should first pin down what kind of hardware platform you are targeting it for, depending on that hardware you get a total budget of different computational things you can do. For example, if you would say I'm targeting the top computational power of most recent high-end PC graphics cards, then you would have a considerable bigger computational budget to do graphical stuff then when say I'm targeting the lower entry level mobile phones computational power.

So as a stock media producer (if that's what you want to do) you need to answer the question (for yourself) what type of client do I want to serve/target. Do you want to make content for developers working with mobile platforms, game consoles, or next gen high end PC hardware?

The conversation does not end here, there are lots of other factors playing part in the consummation of the total computational budget that is available on a given target platform. For example, making a PC game is story A, but now make that same PC game for virtual reality and we have a completely different story B. For example VR content needs more image resolution to render and it need to do it at 90 frame a second versus 30fps, so that will take a big cut out of the available computational budget.

As a stock media producer you can't really play part in the conversation the developer of the eventual end product is performing. He might for example say, I need a characteristic old tree in my game for the slot scene, it is going to play the central role in providing the magic artifact, it may use a bigger part of the computational budget because the rest of the scene would just be some rocks.
In this case this developer would not be served with a very low poly model.

It's the same for a characters and other things. For example, if you make a spaceship the budget may be mush higher because the rest of the scene may mostly be empty space. Now if that same spaceship needs to perform a landing on a very well defined planet, then you suddenly have another conversation in term of hardware budget spending.

So you see, we cannot really make a set of rules defining the total poly count of something, because it depends on the conversation you need to perform in creating the final end product. As a stock media producer you do not deliver that end product, you deliver a part, and you need to (some degree) anticipate on the different scenarios that part can play in the conversation of a given developer.

All that being said, there are some guiding rules that people use that develop for console and the type of games like Call of Duty. They mostly speak about around 30K poly's for game characters, but as I mentioned, it all depends.

As a stock media producer you are most likely to want to produce something relatively general and possibly suitable for implementation in lots of different scenarios and hardware setups.

Personally I try to create things that have maximal visual appeal and minimal hardware footprint.
At the same time I leave some room for the developer to fit things in his conversation.
For example, make 4K textures that may be too big for some applications, but they can easily be scaled down for other needs. You can also (for example) leave in the high poly sculpts, so that they may use it to make a lower or higher poly cage. Maybe you can make 3 different polygon resolutions and make them available separately or in a package.

The bottom line for stock media production is to make things broadly usable or target it very specifically in other cases.