If the button says Install (or Installed - if you installed it, then uninstalled it, the Play Store will still show it installed - you did once), it's free. Look for the "in app purchase" tag under the app's name. That means that probably some features don't come with the free version. (Like Nova Launcher is complete, but it won't bade unless you pay for it - put those little circles with numbers on your phone, email, text, Facebook, etc. icons).
If the button has a price on it (like $1.99), that's how much it costs.
You can install the
Google Opinion Rewards (for free) and give your opinions when it pops up (sometimes a few times a week, sometimes every few weeks). They're short - the longer ones take about a minute. I've "spent" about $5 from my balance and I still have about $15 left. That makes "paid" apps free, legally and safely.
