I think it's usually worth a few million but I'm not sure... usually, when I want to find out the price of something, I just search for recent posts about it in the buying/selling sections. Since prices can fluctuate, it'll give you a notion of how much it can reach and how much it's currently going for.
Usually what I do when setting a fixed price for things is take the original buying price at nook's/able sisters/wherever, and then add half of the original price. If the item is unorderable, I'll increase the price a bit depending on how easily it can be found on here.
So the crown I'd sell for 1,500,000: 1 mil originally + 500k, half of that. Probably a bit less but that's an example of the half price thing.
....usually though I just make it open to offers, which is probably better if you're not sure. ^^;