I use about 30G of a 128GB SSD for Mint and my bulk storage is on 2TB of HDDs. My system is so fast already that I could not justify buying a larger SSD or replacing my HDDs with SSDs.
Because SSD cost is, what?, over 5 to 10 times greater per GB than disks, I decided I would be better off buying 4 disks and using them in RAID 10. This gives very high speed (but not as high as SSD) and high reliability. Indeed, I didnt really need an SSD at all but I wanted one (cause they are sexy) and because I wanted to be able to put the RAID in standby to save power when doing web browsing and non bulk data stuff.
Of course, the days of disks are doomed. It is just a matter of time before SSD costs less than HDD. This may take a decade, tho. In the meantime I reiterate what others have said and suggest an SSD for the OSs and good ol' disks for the rest.