Haven't used their product in years due to terrible unethical sales practices, but when we did, they billed hourly while AWS billed by the second. As such, it was easy to have a monitoring bill that was much higher than the actual resources being monitored (for example, if you had a lot of instances that terminated and relaunched with short durations)
What I found with our customers is that Datadog is about 25% of an AWS bill. With all the addons, etc. I’m still unsure about how the pricing structure actually works since it seems to change from customer to customer. Seems dependent of who your sales person is.