As per other answers in this thread, I can say that it depends. Not to avoid answering the question, but it is very dependent upon the channel(s) that you want to use.
As one of the people on this thread has already alluded to - CPI is only the end cost per install that you end up paying. There are channels, like TV advertising where you will pay to reach a (relatively) specific audience and your spend will back out to an effective CPI (eCPI).
I'll attempt to answer your question, but give you a warning - the CPI is far less important than what you pay vs. what you get from a user engagement perspective. Advertisers usually refer to this as the ROI or ROAS (return on ad spend). The savviest of advertisers are therefore not so much focused on the CPI but rather their percentage of gross revenues that goes towards ad spend. For those who aren't making immediate IAP (in-app purchase) revenues, they still translate the value of in-app actions and engagement to dollar values.
By and large, you'll find that the CPI landscape on the in-app advertising side looks something like this:
Facebook, Twitter and other social networks tend to very tightly control their audiences. They don't want you to bid in realtime on users and therefore ask you to enter your target CPI. Facebook also recently introduced actual CPA bidding - with CPI being a form of CPA this allows you to actually bid the CPI that you're prepared to pay. This may seem transparent, but the hidden cost is attribution, or who they claim the install. Facebook for example will claim any install that occurs when one of their users views an ad - considering that it's easy to scroll through endless ads in the feeds you might not consider this to be far. Unfortunately if you want FB users you have no choice.
DSPs (Demand Side Platforms):
Most exchanges (like Mopub) don't necessarily provide a good interface in order to optimize for buying on a CPI basis, particularly if you intend to optimize towards post install CPA or CPE (cost per engagement) goals. DSPs will place these buys for you and optimize towards your goals.
The challenge here is that exchanges allow DSPs to bid in realtime, which requires a bid to be placed as CPM, rather than CPI. The DSP will therefore optimize to hit your CPI goal, but not necessarily guarantee to hit your goals. Even if they agree to take the risk to hit your CPI goals they probably won't guarantee to hit your CPA goals, leaving you to hold the bag if it doesn't work out. It's important therefore to understand what model they work on and how much you will likely need to invest in order to understand how well they perform.
Ad networks try to more manually match demand and supply and often perform a greater array of peripheral services in order to help you optimize and hit your goals. There are plenty of CPI networks out there, but very few of them will optimize to hit your post install goals.
This is by no means an exhaustive overview, but feel free to contact me if you need more information.