I noticed this a few years ago when we had a tree cut down and I wanted to seed the spot where it had been(along with a hole I'd filled in and a few other bare spots).
For one thing, I noticed it was almost impossible to find just seed for a single type of grass at least at Lowes and Home Depot. Almost all of it was mixes with fertilizer and other goodies added in. And yes, $20/lb or more was the norm.
I bit the bullet, and a single pound covered all I needed to do. 3 years later, we have nice patches of grass in those areas, so I guess it did its job.
Back in Kentucky, we had farm co-op stores under the name "Southern States." A quick Google search tells me they are scattered across Kentucky, West Virginia, Virginia, and North Carolina(although my home town store is no longer there, and one particular one I remember in Mouth of Wilson, VA is no longer there). That's where my mind always goes for this sort of stuff, though. You might go in there and a not be able to get a bag smaller than 20lbs, but it's probably going to be what Lowe's would charge for 5lbs...