While the listed prices for Heroku products are given as monthly costs, we prorate all costs to the second. You can scale down your dynos when they're not in use.
$7 per dyno/month is the maximum you're charged if you run a Hobby or Basic dyno 24 hours a day, 7 days a week, since your last billing cycle.
For example, a Hobby or Basic dyno costs $7/month. Running 1 Hobby or Basic dyno costs roughly $0.25/day. If you use a Hobby or Basic dyno for a Review App and scale the dyno to zero (or delete the Review App) in less than a day, it would cost you less than $0.25.
See Usage and Billing for more info.
For info on how to optimize the performance of your dynos, see Optimizing Dyno Usage.
See our blog post and the Removal of Heroku Free Product Plans FAQ for more info related to the ending of free plans.