How about the North Face Flyweight Rucksack. I now use it as a suitcase, and dump my clothes in the hotel dresser, when I go hiking while on business trips. It can be found for under $50. 32L, weighs 11.8 ounces, and easily fits in airplane overheads and under seats.
Here are some pictures.
I have been using this for a year now and it really holds up. It is not one of my go to packs. But I often use it on day hikes.