I'm not an average user. I'm also running two OSes. I have software installed for both. I need additional space to install additional software in the future. I need space leftover so the drive isn't actually full. A 256GB drive is not enough for that in the first place. Even then it's about useless because most of the software still wouldn't be on the SSD nor would it even remotely fit. I know exactly what my requirements are and anything under 1TB isn't worth it. As it is a 1TB would be mostly full by the time I was done copying over anything which would be better on an SSD. I've already researched what I would need as a minimum and did it quite a while back.
At one point I was going to purchase a Crucial MX500 1TB SATA SSD. I had almost hit the button to purchase it more than once but for whatever reason I simply didn't do it. Then my monitor died. Thus, the money that was earmarked for an SSD went to a new, very cheap monitor since that was more important. Two hours after hitting the button to purchase the monitor my old socket 775 board died. Let's just say it was a good thing I didn't buy the SSD at the time since it would have been useless at best.
You can use smaller capacity SSD's as OS drives and mechanical HDD's for absolutely everything else easier under Linux than Windows due to the fact you can shift your /home partition onto the mechanical HDD leaving the SSD purely for OS duties. No browsing to alternate HDD's when installing software like under Windows, everything just works.
People underestimate just how popular mechanical HDD's still are.