Since December 2020, Amazon Introduced EBS general purpose volumes - gp3.
The bold line below regarding the gp3 price seems relevant to the comparison you did between gp2 and io1:
Today AWS announced the availability of gp3, the next-generation
general purpose SSD volumes for Amazon Elastic Block Store (Amazon
EBS) that enable customers to provision performance independent of
storage capacity and provides up to 20% lower price-point per GB than
existing gp2 volumes. With gp3 volumes, customers can scale IOPS
(input/output operations per second) and throughput without needing to
provision additional block storage capacity, and pay only for the
resources they need.
General purpose SSD volumes make it easy and cost effective for
customers to meet the IOPS and throughput requirements for
transaction-intensive workloads, such as virtual desktops, test and
development environments, low-latency interactive applications, and
boot volumes. With existing general purpose SSD (gp2) volumes,
performance is tied to storage capacity, enabling customers to get
higher IOPS and throughput for their applications by provisioning a
larger storage volume size. But customers want to scale performance
and throughput without paying for storage that they don’t need.
Next generation gp3 volumes offer the ability to independently
provision IOPS and throughput, separate from storage capacity. This
enables customers to scale performance for transaction-intensive
workloads without needing to provision more capacity, so they only pay
for the resources they need. The new gp3 volumes also deliver a
baseline performance of 3,000 IOPS and 125MB/s at any volume size. For
use cases, where your application needs more performance than the
baseline, customers can scale up to 16,000 IOPS and 1,000 MB/s for an
additional fee. This makes the new gp3 volumes ideal for a wide
variety of applications that require high performance at low cost,
including MySQL, Cassandra, virtual desktops, and Hadoop analytics
clusters.
Customers can easily migrate gp2 volumes to gp3 volumes using Elastic
Volumes, which is an existing feature of Amazon EBS. Elastic Volumes
allow customers to modify the volume type, IOPS, or throughput of
their existing EBS volumes without interrupting their Amazon EC2
instances. gp3 volumes are available in all AWS commercial and gov
cloud regions. For more information, please see the gp3 announcement
on the AWS News blog, product overview page, and documentation.
