It is possible to have a DPM less than 1 if metrics are forwarded at longer than 60 second intervals. Sending metrics at intervals longer than 60 seconds will not reduce cost, as billable metrics are determined as the maximum of either the total active series or ingestion rate (data points per minute). When metrics are sent every 60 seconds, the number of active series and data points per minute will be the same. If sending metrics more frequently than every 60 seconds, such as every 15 seconds (4DPM), the number of billable metrics will be higher due to DPM. If sending at a rate longer than 60 seconds, the number of active series will determine the billable usage. Sending metrics at rates longer than 60 seconds will provide fewer data points for visualizations which impacts how far you can zoom in to view changes to metric values.
In both cases, the $8 per 1,000 billable series is only applicable for usage in addition to the included amount of series or DPM. Some plans may include up to 4 DPM for higher graph resolution, and most plans include some amount of active series, though the amounts can vary based on customer agreements and available plan offerings.