Well, that's partly right. Yes, clustrix is more expensive than the open-source databases.

But, companies like Twitter and Netflix certainly have the budget for Exadata (which is what clustrix wants to be when it grows up); they are using Cassandra instead not just because it scales on commodity hardware (the other price factor besides licensing) but also because it works across multiple datacenters which two-phase commit systems can't do no matter how big your budget is, and because its availability (failure tolerance) model is much more robust.

(Many NoSQL systems don't provide these advantages either, which is why lumping all non-relational systems together is usually not helpful.)