You would get the same answer to each question whether you used would or does; both questions ask for the price of the action mentioned. There is a slight difference in the implication of why you're asking, but the information you want is the same.
When you ask How much does it cost to [x], you're simply asking for information. You could be asking out of pure curiosity, or because you want to purchase the item in question, but no implication is made either way.
When you ask How much would it cost to [x], there is a stronger implication that you're wanting to buy the item you're discussing. Instead of a straight pricing inquiry, you're saying "If I were to buy [x], how much would it cost me?" Using would introduces a future conditional, thus the implication that you're considering the purchase (based on the answer to the question about price).
Syntactically, OP's first alternative (Could not you help him?) is more "correct", in that it places the negating not closer to could - the element it most applies to. But idiomatically today we almost always contract that to...
Couldn't you help him?
...even in "formal" contexts. But people often think of such contractions as "informal", and precisely because everyone normally makes the above contraction, the full form as given by OP tends to sound "strange" to the native ear. Thus, when looking for more formal phrasing we're likely to think in terms of discarding the contraction, but we avoid OP's #1 and go for #2 because it doesn't sound quite so odd.
In fact, given a formal setting (lawyer questioning witness, say) there's nothing at all unusual about...
"Is not this the murder weapon, Professor Plum?"
"Could not the gun cabinet have been left unlocked, Colonel Mustard?"
"Was not the dagger kept with the other cutlery in the kitchen, Mrs Peacock?"
OP's idea that the position of not determines whether it's a genuine enquiry or a complaint isn't quite right. What matters is that a complaint would stress the word not - which you can't do if it's contracted. You want that stress because effectively all the other words in the utterance apart from could and not simply describe the current situation (although less common, in most contexts it's possible to stress could instead of not when the intention is to complain about something rather than ask if it's true).
Note that it's not always possible to distinguish a question from a request. For example, given just...
"Could you not do that?"
...it can often be effectively impossible without further context to establish whether the speaker is asking you to stop doing something (complaint) or asking whether you're unable to do it (question).
In short, context and intonation are crucial in such negated constructions. Consider, for example,...
"Could you not do that?"
...which might mean...
1: You're doing it right now, and I want you to stop
2: It's what you plan to do, and I'm asking if you will (or are able to) change your plan
3: It's not what you plan to do, but I'm asking if you will change your plan
(and probably other possibilities)
If it's not obvious how #2 and #3 apply, consider "Could you not work tomorrow?", which could either be asking you to take an unscheduled day off, to work on a day when you normally wouldn't.
Best Answer
If I did not have a car, I would... is correct here. If I had not a car is not, because you aren't negating the right element. "a" is the indefinite article, but it also counts 'one' in a sense. As such, you would need to negate 'a':
becomes
However, in English you would more commonly say "did not have a car", because negating "have" makes more sense, and so "did not have" places the emphasis on that part of the sentence. You could also say "If I did have a car," which would emphasize that also, but we don't tend to in my experience. I'm not entirely sure why the distinction, but "If I had no car" sounds odd to me despite being technically sound, and "If I did have a car" also sounds a little off.