I recommend sampling traces if you aren't. I've been unimpressed with datadog apm, which has no affordable configuration. We've been running our own Jaeger stack with 0.1% sampling, and it's negligible to run compared to datadog apm.

For metrics and logs, sampling isn't so useful, so I don't have a good answer. Datadog has 80% gross margin, so at most 20% of what you pay them is the infra, so you stand to save a lot of money running your own open source stacks if your labor costs would be less than that 80%. With datadog, we have a project every 3 months to reduce usage, so it's not like we aren't constantly babysitting it anyway.