It is 3X more expensive ($0.30/gb vs $0.10/gb for us-east), but it's replicated across AZ's (so is more durable than EBS which is only replicated within an AZ), and you only pay for what you use, you don't need to overprovision the EBS volume to account for peak dataset size.

And since it's shared, you don't need to replicate data across multiple nodes... so if 10 compute nodes needs access to the data set, they can all just read it from the same EFS filesystem, no need to download it 10 times to each compute node.

So EFS can still be very cost effective compared to EBS.