what kind of trekking are you planning to do ? ... for how long
? ....
if it's just to ophir or something, you can probably wing it with a
cheaper pack, but other than that, I strongly suggest you try to
get the best you can afford ....
like I said, I have carried those cheapo stuff before, you can't
load them up, the material's not water-proof, anything remotely
heavy and the harness' creaks ... and your back will hurt
.....
you can swing by camper's corner or something, and see if they have
anything on sale, or you can make do with something smaller, a 60L
pack can last you a long time if you know how to pack your stuff,
but if you plan to be backpacking quite a bit, then it's really a
good idea to buy one good pack to last you a long time than to
waste money on something that'll just break on you sooner or later,
you really get what you pay for ... 
if you need help with other gear (SHOES SHOES SHOES ! very very
very important !!!!! ) and all that, PM me, I can show you pics of
my packs in action too ...
