Multiple times in my career at a new job I've had to build a kind of bootstrapped OSS-based observability platform (mostly a mixture of prometheus and grafana typically) which can come with more overhead than you'd think and doesn't usually provide a lot of the analytical stuff without stitching together a bunch of things. Datadog out of the box gives you everything you can possibly want - as others have stated bills can balloon, for instance if you run spot instances or have a lot of hosts coming up/down I believe it bills you for each one even if it's short lived.

I've been working with an enterprise license for a year and while I don't really hear too much about cost, some simple considerations to the design of the infrastructure it was supporting seems to have prevented a ballooning bill (so far).

So for me, not having the engineering time or buy-in to build a whole home grown observability platform by using OSS tools like this (and all the quirks that can come with them) ends up being a lot more expensive than just sucking it up and buying an enterprise plan. At least so far.

If I had the option to do it from scratch how I wanted, with no time or budget constraints, I'd prefer of course not to be beholden to a major SaaS company that charges for ambiguous things that are hard to predict like "per host", because it's quite easy for these services to bury themselves so deep into your infrastructure that you just bite the bullet on whatever inevitable rug pull or price increase comes next. It has happened to me before managing enterprise Hashicorp Vault.