Found a pair of NOS red seat cushions in my stash that I don't need anymore.  I think my dad bought these in the early '90s and they've just stayed with the car ever since.  What do you guys think is a fair price?

I'm unsure. Its a shame you don't have backs too because that would probably make them pretty valuable.

I have seen some cars with original backs in very good condition, but the bottoms not.  And I have seen some where the bottoms have been replaced but not the backs, like this one on Bringatrailer a while back. 
https://bringatrailer.com/listing/1970-datsun-240z-101/

For the right person, they will be valuable.  But, quality aftermarket ones from Distinctive Industries exist also for about $600 (in black).  

If they are the early type (can you take a pic of them outside the bag?) I'd put their value around $600 - figuring if I had good original backs (rare), I'd be will to spend that much to try to keep the seats original.

Well, the ship has already sailed.  😞  I put them up on eBay last night for $95, and....  they sold in about 10 minutes, lol.  So I'm pretty sure I underpriced them.  Oh well.

Sorry about that. Oh well, move forward...