It really depends on how much advertising dollars you have to spend, and how much money your app makes per user. I'm going to answer this question assuming you've got a free app with in app purchases and advertising.
Here's a few scenarios that commonly play out with FREE apps:
SCENARIO 1. Reskin Game or Low Value Utility App.
If you have low 4 figures to spend on advertising, and your app has a low Revenue Per User figure, the chances are you probably won't be able to make much of a dent buying installs or clicks, and you would be better off investing in life insurance :)
SCENARIO 2: Indie Budget, Social Game or Valuable Utility.
If you have low 4 figures to spend on advertising, and your app has a high Revenue Per User figure (higher than the cost of buying users) then you could be well on the way to a CASH MACHINE that will print money as long as you buy ads. Your low 4 figure ad budget could turn into 7 figures in a matter of months. You could borrow money to speed up your revenue generating cash monster.
SCENARIO 3. Good Budget, Any Low Converting App.
If you have mid 5 figures or even 6 figures to spend, and your app has low revenue per user - then you could spend it on a Burst Campaign, meaning - buying incentivized and non incentivized users through multiple sources on a short burst in attempt to get into to the Top 15 USA chart.
SCENARIO 4. Good Budget, High Converting App.
You have mid 5 figures to 6 figures to spend on an app that generates anywhere from 25 cents to 50 cents or more per user. You have just won the game. Your next challenge is How Not To Screw It Up. That's another question and an entirely different answer :)
In either scenario, if you manage to burst yourself into the Top 15 USA chart, whether through a paid burst or a viral burst (great referral factor, friends want to share the app) or a lucky burst (notch or Justin Bieber tweet about your app)
then you can expect 1 of 3 things to happen:
1) Good Burst Run. You will get hundreds of thousands, or millions of downloads from being in the top charts, and your app will be a success, and after a week or 2, your app will slowly fade away.
-If your app has a low revenue per user (1-5 cents per user) then you just made anywhere from $25000 - $100,000.
-If your app has a decent revenue per user (10 - 25 cents per user) then you just made $50,000 - $250,000!
2) Bad Burst Run. You will get 50,000 - 150,000 downloads over the period of a day or 2, and if your app doesn't have a high rate of sharing/referral factor, you will then drop like a rock down the charts and settle somewhere below the top 100, below the top 100 is also known as App Purgatory.
-Here you made anywhere from $5000 - $15,000 on your 2-3 days of chart topping.
3) Hit App. You will hit the jackpot and become an indie darling success story. You will stay in the top 100 over the coming months and possibly years, get tens or hundreds of millions of downloads, features from Apple, accolades from your peers, and forever be immortalized alongside the likes of Flappy Bird, Doodle Jump, Instagram, SnapChat and other lucky hits that made their benefactors unfathomable sums of money.
So to recap, depending on your budget and the quality and type of app you have to market, there are different strategies that vary from "save your money" to "re-mortgage your house for ads!".
It's not all about buying users either - I've seen friends use some guerilla strategies for high quality indie apps including taking out Facebook Ads that geo target Apple employees in Cuppertino or placing billboards where the Apple App Store Editors are sure to see it.
The feature is not guaranteed with these guerilla strategies, but nothing in the app store is guaranteed :)
I hope this helps.