Logic Apps polling for files or messages can be rather costly if the requirements are such that the file or message must be taken care of within shortly.
Polling every 10th second (one Logic App) for one month will generate 6 * 60 * 24 * 30 => 259200 actions.
The monthly cost for this will be 250000 * 0,000675 + 9200 * 0,000338 = 171,8596 Euro Actual prices 2017-08-11, See Logic App Pricing
Suppose we have at least 10 scenarios like this and the yearly cost will be approximately 172 * 10 * 12 = 20640 Euro
One way to eliminate (almost) the polling cost is to let Azure Function triggers do the job. The cost for that is almost nothing (less than one Euro), only cost for storage account and execution of function when the function is triggered.
So how do we do this?
In this example we want to take action when a new file is stored in a Dropbox folder.
Create a new Azure function and choose External File as input trigger. Select Dropbox as the API Connection.
Other blog posts that addresses the Logic Apps polling case:
Be careful of the Logic App Consumption Prizing model
Save costs by scheduling Logic Apps