At a Glance
$20 - Costs vary based on price of bag and quality of hardware (if you have a bag already, cost is ~$5 for new hardware)
15.4" Laptop, Charger, Mouse, Fleece Sweater, Brick Digi Camera, Multi tools, cell phone, wallet, city maps, spiral notebook etc. etc.
The bag can accommodate 2L bottles (3) and even gallon (milk container form factor).
Step 1: Choosing your pannier style
Of course, there's your DIY standard "bucket pannier" made from old cat litter buckets (or similar) as seen here or [http://web.archive.org/web/20050208161130/http://members.rogers.com/bphuntley/BikeBucket.html here] or even here. While utilitarian and high capacity, these lack a certain sense of sexiness. You can make the panniers seen in this instructable for the same price as these bucket panniers if you use some recycled material.
There's also the surplus army bag conversion - [http://web.archive.org/web/20041020064156/http://www.twowheelfetish.com/Ezine4/diypanniers.html as seen here by Peter Moore].
You could, gasp!, buy panniers... Arkel makes some great bags - the one closest to my DIY version is the Arkel Bug Pack. It's convertible - it goes from pannier to backpack easily. It also has a quality engineered mounting mechanism. The price isn't something I get excited over, $165... But I've met a happy customer, and will vouch that it is quality.
Photo Credit (Buckets): Brian Huntley