On iOS I use CPI but CPM on Android. For CPM, I try to keep it at $1.50. And the min is $250/day. My last creative had a conversion rate of 0.81%. So about 8 installs per $1.50 or 18 cents per install in US and Canada.
On those installs, I got the following retention:
24h retention of 17.5%
72h retention of 10%.
I put hours because I get the values from Tenjin and Tenjin calculates retention for hours after installation. So 3-day retention is 72 hours from the time of installation.
Also, note that the install rate in the Chartboost console is for the install rate of clicks not of total impression. For the campaign above I got a click-through rate of 16% and an install rate of those click-throughs of 5%.