Yeah, combined with the above comment, I think this is a good idea. This was actually my initial thought (but with 2 friends) early on, but I wanted to experiment with the idea of only having a paid version because I believe paid apps get more usage (I might be crazy here).
From my experience free trumps paid unless the paid app is promoted by apple/lots of blogs. You may get more return users with a paid app (since they shelled out the cash they will probably want to get their money's worth) but if you're trying to build a user base and especially if your app relies on one, which yours seems to, free is the way to go.
