High Sierra Topix

http://www.zimmerbuilt.com/

Custom packs, well-known on backpackinglight for great customer service.

Also http://kifaru.net/, https://seekoutside.com/lightweight-backpacks/

Dave

Cilo gear makes good stuff, but they are more Alpine oriented... Pretty burly. Also not cheap (I don't know what you mean by affordable... You want US made, it'll cost more)... but they are super adjustable and last. I have put mine through very tough conditions and it has held up great.

Alpine Luddites in CO also makes alpine style packs.
Rivendell Mountain Works for the old Jensen-style frameless packs.
All proudly made in the USA.

I've got both the Kalais and Aquilo packs made to order by Matthew Lagos-Rivera at Elemental Horizons and can recommend them without reservation for build quality and design, carrying loads up to 45 pounds in the Aquilo. Will Rietveld over at Backpacking Light reviewed the Kalais as one of the best backpacks he's ever tested. Matthew has recently fine-tuned the suspension system and I notice he's got one of the previous generation Aquilos (same as mine) on sale for $190.00. That's a steal.

Some good suggestions so far. However, note that neither Gossamer Gear's nor Six Moon Designs' backpacks are sewn in the USA. Years past they were, but times have changed.

Personally I'm very intrigued with Elemental Horizons especially with their newer suspension. The Kalais XT looks sweet.

I hesitate with my suggestions because I wouldn't necessary call them affordable. Surely a great value for what you get, but cheaper options obviously exist. So with that:

-Hyperlight Mountain Gear (HMG). A 3400 Porter (maybe w/ the frame sheet option to up the load carry).
-Katabatic Gear. Either a Helios or Artemis. 40 lbs. would be the most I'd expect to carry in reasonable comfort though.
-Superior Wilderness Designs. Similar to HMG. Again, no way would I carry 50 lbs. in it.
-Seek Outside. Divide 4500. OK, not truly an internally framed pack, but not a typical external either. It'll carry whatever you can handle. It's worth a look.