Apples official way ......
You need to use NSNumbers and NSNumberFormatter to convert the value to a currency and return a string representing it. This can be used in our labels e.t.c. someDouble refers to the number you wish to convert to a currency format.
NSNumber *currencyValue = [NSNumber numberWithDouble:someDouble];
NSNumberFormatter *formater = [[NSNumberFormatter alloc] init];
[formater setNumberStyle:NSNumberFormatterCurrencyStyle];
NSString *currencyString = [formater stringFromNumber:currencyValue];
NOTE: you may need to play around with the setLocale
to get it to display the correct currency for a given region.