You can see the cost per month (or per year) on the Construct website when you go to purchase it.
If you paid for 1 month and exported a game to android and uploaded to the android store, then after 1 month, your game will still remain on the android store forever, but your access to features in Construct will go back to the "free" version, meaning you will have restrictions such as limited number of events, and won't be able to export to android until you pay for another month. You can still open your project and test it and make other changes, but will be limited.