Calculate the amount of Primecoin(XPM) mined/created so far According to Wikipedia:

http://en.wikipedia.org/wiki/Primecoin

the reward given for finding a block is 999*(difficulty**2). So does that mean that you need historic data on the difficulty in order to calculate how many coins there are?
Yes, unlike Bitcoin you can't deduce the total number of coins in circulation from the current block height, you need historic difficulty data. Of course, there should be services that have already computed this and present it publicly.