You should try to distinguish between "this can't be answered even in principle" and "this may be very difficult to answer, and require information that is obscure or not public". The former is almost always a bad question (and should be downvoted and closed if it can't be improved), but the latter is often a good question, and some of the most interesting questions on the site are of this form. Indeed, if every question on this site were very easy to answer, we would not have much reason to have a site in the first place. It is difficult to draw a fine line between the two cases, but if you can reasonably imagine that some answer (even an incomplete one) could be present, even if only in some obscure (possibly untranslated) source or only to people working close to the industry, then if it is within our scope and of general interest to anime/manga/etc. fans it is probably a good question for this site.
For the question you are asking about, I think it falls in the latter category. I don't think the OP wants to know exactly how much everything on https://vndb.org/ cost to produce and how much profit they made; this would indeed be unreasonable. Just like with anime, this information will not be public for most VNs, but that doesn't preclude giving an approximate range with a few examples. And it wouldn't be very surprising if there are a few examples are publicly available. Certainly, more of these figures are private than not, but just 2 or 3 would be enough to write a good answer to that question.
Actually, some numbers must be known to a fair number of people, because somebody has to fund these projects. Even if the funding all comes from internally, there's presumably someone knowledgeable about the budget on the staff of every major VN studio. That's actually a fair number of people overall, and only a few of them would have needed to say anything public to get a pretty good idea. In fact, I think I have even read interviews directly addressing this issue (as I stated in a comment on the question) and it would not surprise me if there were more. And even if we can't find any examples, we can certainly put bounds on the cost by finding VNs which did turn a profit (e.g. pretty much anything with an anime adaptation) and looking at their sales numbers.
So ultimately, just because a question can not be answered totally does not mean it can not be answered well. I think the question you have linked to is quite a good example of this. Answering it is likely to be fairly difficult, but a good answer to it would be quite helpful. You might think of this as a "high risk, high reward", but actually, because space on this site is of no shortage and the question is not likely to attract many spam or low-quality answers, it is low risk and high reward. If you still don't think the question is good, you can downvote it and suggest improvements in the comments, but personally I think this question is actually quite good and interesting. I could probably even give an okay answer myself, but I'd rather leave it for someone more ambitious to answer.