Camera bags are great for protecting your gear, but if you aren't a professional photographer spending cash on a dedicated bag can be excessive. Luckily, with the right accessories any bag can function as your camera bag.
In this video from photographer Caleb Pike, we see how he adapts a cheap canvas messenger bag. The secret is really in the accessories. Lens cases are an obvious choice, but you can also use those lens cases for any loose batteries, wires and other gear that might be rattling around you bag. (And you can even make a rudimentary lens case yourself.)
What's more interesting and new to me, is the Domke wrap. It's a padded covering that you can wrap around anything that needs extra protection, from camera bodies to monitors and anything that will fit in your bag. (I've previously wrapped my camera in a sweater when carrying it in a bag... which worked, but left me chilly).
Pike also suggests Porta Brace Stuff Sacks to store any extra cables or batteries; those look great but are pretty pricey, so I'd opt for any cheap pouch.
Toss it all in any bag and your gear should be safe and ready for shooting when you need it.
How to Turn Any Bag Into a Camera Bag [DSLR Video Shooter]