With our power rates payback is about 5 years for the SSD's direct power use, if power prices don't go up. Savings in air conditioning will be about double that, in Arizona we spend a lot keeping cool. So figure breaking even in a couple years.
Add in all the other goodies you get with an SSD and I figure it was a good deal for me since it was a "free" upgrade at worst. Besides all my other boxes that get used regularly have had SSDs added as their primary/only drives and the pfSense box was the last spinning drive I had for other than bulk data storage.
My pfSense setup is pretty basic, no packages or fancy filtering so load on whatever drive is in there is low so there is little gain from the traditional benefits of an SSD. Speed? Who cares if it boots faster, that is likely to be a twice a year event when I shutdown everything here for a dust blowing session or if I need to reboot after an upgrade. Noise? Never heard much noise from the old pfSense box and drive, an IDE one dating back to maybe 1999 in a 2001 Dell GX100.
I had to put some type of drive in the refurbished system I got for my new pfSense install, it had to be SATA and I didn't have any likely candidates in my parts bin so I had to buy something making the power use payback about the only real factor in the SSD/Rust decision for me.