Unless you are well funded, I think you have to proceed and gear up drive your own sales in the app stores by bootstrapping it. When I say by bootstrapping it, I mean you have to do a lot of different things to try and push awareness of your mobile game and build excitement so as not to solely rely on the number of downloads and the promotion of the game within the app stores themselves. Our game didn't hit the app stores yet, but I can tell you what we did because it got us in the press and it got us a small pile of investor cash so we are definitely doing something that works in the hype department.
@bgillisp is correct with his sales stat, but that's the average across all the games I believe. If you are building something unique and if you are fiendishly and obsessively going to bring it to market, then I don't put a whole lot of weight in average statistics because when you are all in, you will do whatever you need to in order to get sales and make a good game.
If your game is good enough, and you can build the required hype and public awareness, your game could do well above average even if it comes out of the gate without much fanfare. Ideally you want it to launch with great reviews and major hype out of the gate, but this is not always realistic, so having all these other marketing resources in place will be helpful regardless.
This is what we are doing or have done:
1) Come up with a platform or some unique feature that nobody has. Ours was geo-based and time based content and we built a platform that serves up that functionality to our game. But it can be simpler than that. It just needs to be some kind of unique hook. If you can design an argument around the hook that the hook is unique enough to generate revenue, that would also be helpful.
2) Hype the game and the hook on social media, reach out to incubators, and local investor & development resources in your local community. Your local community is going to be your greatest asset, especially if the hook is unique enough. You will be able to get an incubator to assist you, and local or regional media and press involved to spotlight it. Also reach out to your schools you graduated from, they often can help get your game and the related hook in the media and also spread awareness.
3) This will put your game out there so that sales do not necessarily reliant on the app stores to promote it.
4) Get some influencers to review and hype your game, build a website that showcases it, and do everything you can outside the app stores to promote and market it. Social media and web presence should be big. Then when the game is done, you can point to those external resources as you run around and further advertise, showcase, and hype it.
I'll be happy to share more insight with you with respect to what we did and what we are doing. I think if you can make a game with the right combination of things, it could sell quite well. It cannot simply be a good game, it has to have some other things to it. I think an average game plus some unique extras will make it sell better than being a really good game with nothing else. The gaming market is not a single customer base.
There are a whole bunch of different types of people who play games. Understanding the market and who these people are, what moves them to purchase, what games they play, and how they define a good game is critically important.
I've done this research, and I have found that there are some unique demographics of gamers who are not the type of gamers people usually thing of when they hear the word "gamer". Those different markets of game purchasers are what led me to the pathway I am on with my game.
Find out who your target markets are and make your game so it meets their needs.
Don't worry about the bigger companies who buy reviews and pay for likes and clicks. They will always have the big cash to do that and you will never have that amount of cash, so do your promotion organically, and get people to know about your game outside of the app stores. You want them opening up the app store and going in there specifically to buy your game, that they found out about somewhere else. As long as the reviews are decent and not bad, they already made up their mind so there is nothing to stop them unless it has all or majority bad reviews. If you take pride in your work, that won't happen.
Thats what I am doing, and I believe it 100%. I am banking my game on that model. So I'n my opinion and based on everything I've researched and done to date, I would recommend the pathway I described above.