Rosedust is one of the more common flutters and she is in her US box, which is also more common. I would say $100 max for her, to be honest. Flutter prices vary, especially MIB, but the ones that sell for the highest price tend to be the 2nd year release MIB as they come up less often. Rosedust was on sale both years in all places that had flutters, so she's not that rare in my opinion, compared to the others.
The ones in UK box also often sell for more money, because they have the puffy stickers which people often want. The US ones don't have these. UK boxed ones also come up less frequently.
So I would definitely hold off on this if you can't get a good price on it. Rosedust will come up again, and there's no sense in shelling out $200 if you don't need to.