It is! At least in terms of raw minute pricing, though Depot builds are often faster than Actions, around 2-5x for Intel builds and closer to 10x faster for Arm builds, which affects the comparison a bit.
Our biggest cost at the moment is persistent disks. Each project gets its own SSD(s) that live for the lifetime of the project, and that's a major part of why Depot builds are so much faster and support BuildKit features that require persistent disks (e.g. RUN --mount=type=cache). We have some ideas for how we might improve this in the future, keeping the same or better performance profile but avoiding the continuously-provisioned disk, we'll see how that goes.
One note on "per project", each Depot project represents a pair of Intel/Arm VMs, each with a 50GB SSD. But there's otherwise no restriction on what you build using those projects, you can use a single project with multiple repos/Dockerfiles/whatever. Projects are the access control boundary, and building multiple disparate images using the same project would cycle the cache faster, but otherwise there are no limits on concurrency or what's building inside those VMs.
Our biggest cost at the moment is persistent disks. Each project gets its own SSD(s) that live for the lifetime of the project, and that's a major part of why Depot builds are so much faster and support BuildKit features that require persistent disks (e.g. RUN --mount=type=cache). We have some ideas for how we might improve this in the future, keeping the same or better performance profile but avoiding the continuously-provisioned disk, we'll see how that goes.
One note on "per project", each Depot project represents a pair of Intel/Arm VMs, each with a 50GB SSD. But there's otherwise no restriction on what you build using those projects, you can use a single project with multiple repos/Dockerfiles/whatever. Projects are the access control boundary, and building multiple disparate images using the same project would cycle the cache faster, but otherwise there are no limits on concurrency or what's building inside those VMs.