If I buy Pages from the Mac OS app store, do I then have to buy it again in the iOS store?
I'd like to not have to buy it twice.
Yes, you do. They are two entirely separate apps for two radically different devices.
OS X and iOS are two different operating systems so it doesn't work. The one for the iPad is a simpler (smaller file) than the one for the mac. So yes you need to buy a copy for your mac and another for your iPhone/ipad.