Check out the Tamrac Evolution backpack/sling bag. I have the Evolution 6, it's a little smaller than the Evolution 8. I can fit my T2i with my 17-50 lens attached, the 55-250, 50 1.8, speedlite, point and shoot, etc. etc. all in the main area. Then there are pockets on the sides and an additional area on top for chargers, filters, and other gear.
This bag is a neat concept. It can worn as a backpack or easily switched to a sling bag for either shoulder just by moving the straps. I like to wear it as a backpack since then it allows me to get to both the camera and the extra lenses by slipping off either shoulder strap and swinging the bag around.
I had been set on the Lowepro sling bags mentioned above until I spotted this one at the camera shop.
Edit: Well, shoot. It looks like it is above your $100 budget. Personally, I think it is worth stretching a little, but maybe you can find it on sale somewhere. If not, I hear great things about the Lowepros.