First thing that comes to my mind when considering a "hard case" is Pelican,Starlight or Storm.They are more than $100 though.There are cheaper cases like SKB that you could probably shove a bunch of stuff into along with your M4.
I would just save up a little more cash and get a Pelican/Starlight/Storm.Why skimp on a case for your $1K+ toy ? You can custom cut the foam and they are bombproof/waterproof/airline baggage handler proof.
I was considering a Pelican or Stralight,but decided to go with a basic gym bag with my M4 broken down and wrapped with a Army wool scarf.I have a small Army canvas shooters bag that I also throw in the gym bag that holds 6 30 round mags and maybe 700 rounds in boxes in the center part.I put my optic in a small hard sided camcorder case I got from Best Buy for about $15.00.After the basics in the gym bag,there is till plenty of room for other junk,but for all my cleaning stuff/parts,I just have a tool box jammed full of everything I need,this I keep in my trunk.
Here is a thread talking about Pelican and Starlight.As far as cheap hard cases,it seems it isn't discussed here much at all.Reason being (I think) is that people that spend thousands on a weapon, don't skimp when it comes to a case to protect it.