Hobie,
You will not find a decent backpack for $150. unless it's on a big sale.
I suggest you buy a good book on backpacking tat will tell you what to look for in a pack AND other gear.
What you should consider:
1. internal or external frame?
2. Capacity (medium is around 4,000 cu. in. or 50 liters)
3. Torso length MATCHES your torso length (Some internal frame packs have adjustable harness lengths)
4. Padded hip belt size and comfort.
5. Lift straps above the shoulder straps.
When you think you have found the right pack take it home, load it up with 45 lbs. of weight(books, etc.) and walk around for about an hour, adjusting to fit. If it is still fairly comfortable after an hour keep it.
Eric B.