Back to the higher priced options, you could get a
5.11 Select Carry pack. My 11.5" AR won't fit, but the 7.5" upper will, as will the 5.5" 9mm upper. Look around and you can find a better price than the one shown at the link posted.
It was obviously designed for something the size of an SMG, or very short SBR. The Uzi or an MP5 would work well, as would a Krink AK folder.
http://img.photobucket.com/albums/v6.../100_0874a.jpg
Actually, I've found that if I put the 11.5" upper on a pistol lower, the buffer tube just hangs out the bottom, looking like a tennis racket, or something similarly benign.
http://img.photobucket.com/albums/v6...e/100_0880.jpg
A 7.5 fits, but you wouldn't want anything longer than an A2 flash hider, and this is a little slower to remove due to the length than something a little shorter. I'm half tempted to just get a muzzle nut to cover the threads and not add any length to the barrel if I were to pack this around in the bag.
http://img.photobucket.com/albums/v6...e/100_1166.jpg
http://img.photobucket.com/albums/v6...e/100_1167.jpg
Also, you can break down a 16" rifle, and carry that as well.
http://img.photobucket.com/albums/v6...e/100_0876.jpg
The thing is durable, and carries well. It has a wide padded shoulder strap that is comfortable to use. I used this to lug my gear around for the Knob Creek subgun match. It is well made and worked very well, even though I had it packed completely full with gun, ammo, mags & gear.
I also use an Eagle case, that I think was in the $80 range. It will fit the 11.5" gun assembled, or can hold a 16" AK underfolder.
http://img.photobucket.com/albums/v6...J/DSC03352.jpg
The ever popular tennis racket bag actually works pretty well too, and you can find a good padded racket bag for $20-$40 if you look around.