Usually both, but they show up in different places.

You need synchronization semantics one way or another. Even in actor systems, "send" is not magic. At minimum you need publication of the message into a mailbox with the right visibility guarantees, which means some combination of atomic ops, cache coherence traffic, and scheduler interaction. If the mailbox is cross-thread, fencing or equivalent ordering costs are part of the deal. Copying is a separate question: some systems copy eagerly, some pass pointers to immutable/refcounted data, some do small-object optimization, some rely on per-process heaps so "copy" is also a GC boundary decision.

The reason people tolerate message passing is that the costs are more legible. You pay per message, but you often avoid shared mutable state, lock convoying, and the weird tail latencies that come from many heaps or stacks aging badly under load. Fragmentation is less about one message being cheaper than one fence. It is more that at very high concurrency, memory layout failures become systemic. A benchmark showing cheap fibers on day one is not very informative if the real service runs for weeks and the allocator starts looking like modern art.

So no, I would not claim actor messaging is generally cheaper than fragmented memory in a local micro sense. I am saying it can be cheaper than the whole failure mode of "millions of stateful concurrent entities plus ad hoc sharing plus optimistic benchmarks." Different comparison.