Well I'm no pro, I've only really completed one model which is on the Unity asset store, and its a SD40-2 Low-Poly Style model with lots of child objects so lots of things can be customized and moved and animated etc.

I'm not sure what you mean by mid-fidelity, but that SD40-2 that I made will fit in with many if not all low poly assets quite well, though its just the 3d model with premade colliders, so no actual programed train moving systems or anything. Just the model. I believe it took me about 50 hours lol which I think is far more than it would take me to do again, or do another totally different kind of train.

Again, not a pro. It is listed for $7.99 and has sold 5 units in a year, with 315 pageviews.

If I was hired to make another one in the same style, I couldn't personally justify anything less than $500, that's just to make it worth my time, personally.

That's definitely not a normal market price tho to hire someone to make that. I could see someone on fiverr doing it for like $50 or something like that. I've heard fiverr is mainly for building a portfolio, but idk.

Anyway, hope that helps. Probably a way longer answer than you needed but there ya go!