SSDs do provide apples-to-apples performance improvement over HDDs (even 10K RPM drives). One challenge with AMP-based packages such as Drupal is that they are not often
I/O-bound unless precisely tuned for particular workloads. As such, 'out of the box' performance gains might not justify the increased cost.
Properly-tuned Drupal deployments which stress I/O subsystems will likely see greater SSDvs- HDD performance increases than detailed in this report.
ZFS Hybrid Storage Pools provide a more cost-effective option in which some SSD speed advantages can be realized without the full upcharge of SSDs.