This is actually a pretty complicated topic, there are cheap duffel bags that you can get for $25 at Columbia for instance, but if you look at Outdoor Gear Labs, you can spend a lot more to get a lot more.
The truth is that I've had the same duffel for literally decades and it is still holding together. This is the most basic sort. A single zipper in the middle, but the better ones have lots of pockets and have a U-shape, so it is easy to the load.
The best things about a duffel compared to say a rollaboard is that it is light (5 pounds or so vs 10 or more) for something of the same size. A large is really large over 65 liters.
The best ones is the Northface Base Camp which lists for $149 which is pricy, but you have a bunch of options:
- REI. You can get closeout model for $75 but these are a little shall we saw interesting looking. The other choice is that right now you can buy a full priced item over $100 and then get 10% back from the REI Co-op rebate and then another $20 off, this brings the cost down to $150-15-20 or $115.
- Moosejaw. This is another option, they have a little complicated program, but if you are student, you get 20% off and then you get 10% back in Moosejaw points. Plus if you use cashbackmonitor, they have a 16% rebate. With these retailers, If you need two, it makes sense to buy one, then get the points and then use that to pay for the next one, so effectively you get $150-20%-16%-10% so it is effectively $96 for the product. Still expensive, but a few clicks and things are a bit better.