Disks are more expensive for most server use today than SSDs. You can't just look at cost per GB. You need to look at cost to process per GB, and then the vastly increased IOPS and transfer rates provided by SSDs makes them cheaper for most applications where data is regularly accessed at this point.

There are niches, such as long term archival, backups, etc. where disks may still be competitive, but even those are rapidly being eroded.

For things like web servers or database servers, I'll never provision another machine without SSDs.