I've been using one of these for years. Dead simple. Has held up well: Inner lining has been frayed from contents, but nothing that can't be easily fixed. Even with the inner lining frayed, it's decently waterproof. Seams and fabric are still ~100% after 5 years. It's moderately comfortable. Not as nice as the $200 packs, but not terrible either.
Found it at army surplus.
https://www.foxoutdoor.com/ItemDetails.aspx?cid=1&ccid=1&cscid=908
Was $50 on sale for 45L
in ballistic nylon. Had it 5 years, expect at least another 5 from it. One of my favorite purchases.
You can add on modularly. They've got velcro strips on the back and top, two ALICE-style "belt" strips on the top, plus you can attach caribiners to some plastic hooks along the back for lighter hang-ons. Not for those who want something with 50 different special compartments, though. But then you can add MOLLE/ALICE/etc gear to this pack with ease, though it might not look so pretty.
They're not the lightest, but not as heavy as cotton either, so I felt it worth the extra few ounces to save so much and never have to worry about snagging it on a stick or rock. This stuff doesn't need ripstop, it just doesn't rip in the first place.
They've got a 25L version for $35 I just bought for the missus. Guess we know who's carrying the lion's share of weight, eh?