I bought this Port Chicago Eco from an online store for £20 and it is a good buy in my opinion,
You can get second hand ones from Amazon for less but I'm happy with my purchase, it arrived well packaged within the seven days the site promised to deliver.
I bought it from laptopsdirect.com.

The bag is comfortable to carry but you have to watch what you put in there as it does carry quiet a bit and if you have to walk through town with a heavy bag it can be a bit inconvenient, but having this in my car is great as I can take everything I need out with me.

The case is made from 24 recycled plastic bottles and a mix of Polyester and canvas, so it's waterproof and can be wiped down with a damp cloth, the bag is cushioned on each compartment so that things inside don't get banged about and the sleeves inside the compartment hold things in place.
It has three compartments, all that zip up with a sturdy zip. The middle compartment is where your laptop fits snuggly inside and is held in place with a band.

On the one side there is another zip up compartment for keeping notebooks or files you might need to work from and the other side has another zip compartment for more storage, this has a smaller compartment attached to it that also has a zip.

There is plenty of room for everything you might need to carry and the bag has a handle as well as a shoulder strap that you can fit if you want to sling it over your shoulder.
This is quite comfortable and I prefer to use the strap and not the handle as I sometimes load the bag up with lots of files, paperwork and other work related stuff and it's easier to carry using the strap.

The Dimensions are :- 480 x 350 x 80 and it weights 1.3Kg
A good durable laptop bag that I can recommend.