1. How to do it? My thought is the answer involves building AI's that are much more than just LLM's. You need a system that has knowledge and a world model and common-sense reasoning, etc. When you're doing nothing but "predict the next token based on statistical patterns" then yeah, you're going to get hallucinations.*
2. How much will it cost?
3. How long will it take?
And so on.
* Let me add that that's a very "off the cuff" view of my current thinking on building AI's that are less susceptible to "hallucinations" (or "confabulations" if you prefer). But I'm not saying that is the only way, or that it will ever be possible to construct AI's that never make mistakes at all. After all, our current "gold standard" for intelligence to compare with (eg, human intelligence) is quite prone to making mistakes. So I'm only talking about getting to a level that eliminates the really egregious hallucofabulations (to coin a word).