That's about what I've been getting recently for my game. About 40% of the clicks from the ads result in downloads of the game, then about one out of 20 of the downloader's actually end up paying for a subscription. It seems getting people to click on ads and getting people to download (especially when it's a small download) are the relatively easy parts. Getting them to create an account once they've downloaded and getting them to pay once they've tried it are much harder.