It's great to see this discussion going on in here!
I too have been doing car renderings as a "hobby" more or less. I've been doing work as a graphic designer/illustrator for about 20 years now!
I've drawn everything from wildlife to aviation to automobiles. In all this time I have yet to develop a "price sheet" that will work for every project!
Most often it is "customized" or "negotiated" in order to fit the expectation/budget of the customer.
Ultimately, the final price is determined by you! But, just like other services, it needs to be a fair price for the skill level you can offer. Try to avoid quoting a price that is so low that you regret having to complete the project. You'll be miserable and your work may reflect that as well.
It also doesn't hurt to just simply ask the customer what price they had in mind.
You might also consider offering different options from "top of the line" to "very simple" and maybe a few options in between.
I've also done some rend'gs for speed parts as well!
I guess the main thing I'm trying to say here is to be flexible on everything but the pride in your work!
Well, it's late and I'm tired but I've been following this post and felt the need to contribute. Hopefully it'll make sense in the morning!