What is LTV (lifetime value)?
LTV is the total revenue an average paying user brings over the whole time they stay, after the store's commission.
For a subscription, the simple formula is the net price per period divided by churn per period: a $10 monthly plan with a 15% store fee and 10% monthly churn is worth about $85 per subscriber.
LTV sets the ceiling on everything you can spend to win a user. Multiply it by the share of installs that become paying users and you have what one install is worth.