How about this:
I use that backpack to carry my whole technicians' toolkit (laptop, power bricks x2, ext. HDD, etx.FDD, two rolls of handtools, more handtools, spare leads, mice, stacks of discs etc. etc..)
It's about 20KG of stuff and despite being totally full the rucksack has never let me down. It's fairly comfortable too having a comfortable strap with shock-absorbers.
I bought it from play.com for £36 but it gone up to £43 now. Still probably the cheapest considering that's inclusive of postage.
http://www.play.com/PC/PCs/4-/949380.../Product.html#