This really depends on the amount of hot data you have - and you can only determine this when you've deployed the solution.
When virtualizing databases on your workload, i would recommend to
calculate at least with enough SSD storage for these databases -
having many IOPS here is not uncommon and may be a starting point. But
that's really just a very rough rule of thumb.
In my opinion, a much more important factor is not only the amount of SSD storage you have, it's also the amount of disks.
Storage pools performance is heavily dependent on the NumberOfColums you are using. As this value cannot be changed on existing virtual disks you propably want to get a good value when creating the disk.
Recommendations here are to use NumberOfColumns between 3 to 4 - so either 3 or 4. Having more columns means more speed but increases latency slightly.
Another important factor is the interleaving size. There are recommendations (what we're also using) to set a reduced interleaving size with 64k for hyper-v workloads. Default is 256K (at least until Server 2012 R2)
To reply your actual question:
It may be better to get at least 4 or 6 SSD drives and put them on a pool with at least 4 or 6 HDD drives. When planning for scale out, use 6 SSD + 6 HDD and add more HDD drives if you need more storage and optimization doesn't show that you're on the limits of your SSD drives.
Regarding pricing the solution may be in a similar price range than many PCIe cards but be more scalable and even faster.
A nice side effect is that you'll propably have more space on the SSD pool available since the multiple SSD drives provide propably more storage than the PCIe drives.