LTDs have a base price of around 300k, with slight variations due to local demand levels (unlike the other gem types, LTDs have a large enough base demand factor that the number recently sold to a station can somewhat affect their price)
Various BGS states affect this - a combination of Investment, Civil Liberty, Pirate Attack and Expansion will raise it to around 1.1 million, though that's quite a rare combination.
It's possible that there's a fifth BGS state which can be added to that list (or perhaps replacing one of those four) to get 1.7 million - but I don't know what it is. Can you remember *where* you sold them, and exactly when?
As to whether the prices will come back? Most BGS states are directly and only caused by player activity, so you could set something up. The exception is Pirate Attack, which appears at random for a few days and then disappears again - but you could find a system where that had just started, and then push the other states in before it ended. If you just wait around for it to happen, you're unlikely to see those prices just happen to reappear as a result of other player actions.
I certainly wouldn't hang on to them in the hope the price goes back up in the next day or two - that's extremely unlikely to happen.
The effects of BGS states on trade prices changed significantly with the 3.3 release and because the "high" economic states currently tend to be permanent (and the "low" ones non-existent) it's a bit early to re-analyse what the optimal combination for each trade good is ... and the effects of overlapping states makes it a bit tougher to isolate them, as well. It's on my list to do, though.