Cost Per Install (CPI) is the average amount you pay to acquire one install through paid channels — the benchmark organic ASO is measured against.
CPI is paid acquisition's headline number: total ad spend divided by the installs it produced. It varies enormously by channel, geography, and category, and it tends to rise over time as competition for the same users increases. Teams track it because it sets the price of growth — if your CPI is higher than the lifetime value of the users it brings, you're buying installs at a loss.
ASO matters precisely because it has no per-install cost. A keyword ranking or a higher conversion rate keeps producing organic installs month after month at zero marginal spend, which steadily lowers your blended CPI across paid and organic combined. The two also reinforce each other: a paid burst can manufacture the install velocity that lifts organic rank, and a strong organic base means you can afford to pay for fewer installs.
Example
A startup paying a $4 CPI invests in ASO; six months later organic installs have doubled, and because those came free, the blended cost of every install across the business drops sharply.