I really like my Lowepro Photosport 200 AW when I need to carry a fair bit of gear. However, it will probably cost you closer to $150. You might be able to find it for less on eBay.
Lowepro Photo Sport 200 AW Backpack (Black) LP36353 B&H Photo
When I was looking for a camera backpack, I looked very closely at the weight of the bags, as well as other features, and this is one of the lightest that had all the features I wanted (including a raincover, side access, tripod pocket (or straps below), room for accessories, etc. I have a very bad back and this pack is super-comfortable even when fully loaded. The weight is well-distributed and the waist strap makes a big difference in lifting the load off my shoulders. When I'm photographing rescue animals (farm sanctuary or at adoption drives for dogs), I'm moving around a lot, standing up, crouching down, moving from side-to-side - and I have nowhere to leave my stuff to one side - so it's all on my back. It's also a great bag for travelling (on the plane and at my destination - it went to Borneo with me, among other places.
I usually pack it as follows: the padded, side access camera compartment holds my K-3 with either the Sigma 17-70 or Pentax 55-300 attached; adjacent section in camera compartment holds the other large lens plus the 77mm. In the top-loading compartment, the flash and possibly a 2nd small lens sit down at the bottom of the pack and held fairly snugly by being adjacent to (but on the other side of the camera compartment). I also fit items like cables, Gary Fong light diffuser etc in the top compartment (could also hold your filter pouch, wallet etc). The outside open pocket can hold a flash bender (if I don't bring the Fong). There is a zip internal pocket for small items I want to keep secure (e.g., wallet, passport, phone). A zip compartment on the top of the bag holds extra memory cards, brush, lens cleaning cloths, spare batteries, business cards, etc. There is a pocket on the side that, with a little ingenuity, you can rig to hold your tripod (or you can attach your tripod to the bottom straps. There also are two very small pockets in the waist belt that you can use to hold memory cards or a very skinny wallet - too small for today's larger phones.
The only quibbles I have with it are as follows: It's pretty easy to lose (in the bag, not forever) the stuff you put in the top-loading compartment because it's not subdivided. (That's the trade-off I guess for having a flexible, large compartment in a lightweight bag.) I deal with it by putting some of the easy-lose-small stuff in a small, lightweight mesh bag - easier to find. I also wish the bag was solid black, instead of black and grey, because the bag looks very sporty. That's fine for most purposes but sometimes you want a bag that is more discreet and professional-looking.