Honestly I don't know - I lost 70 lbs a few years ago, and anything I put on my back is gravy these days. The Kelty MAP is made of Cordura nylon so is probably heavier than equivalent back packs made of less durable nylon, but not inordinately so - its a low profile pack that packs the majority of your gear on your waist and not on your shoulders, so all in all it doesn't seem very heavy to me empty or loaded. An email to Amron Intl may get you an answer - I'll guess maybe 3-4 lbs. I find Kelty brand packs aren't in the lowest weight category, but they are well built and I like to carry them.

One other bit on your requirements, if you're packing 40 lbs in a BOB I submit you're over packing: I can pack full overnight gear and 3 days food in a Kelty. 40 lbs may be at the upper limit of many packs, particularly lightweight ones - the Gregory Tarne I cited weighs probably 2 lbs, is a great pack, but you would be maxing it with 40 lbs. I had similar issue with another Gregory lightweight pack - it was terrific with 25-30 lbs, but a bear to lug around with 35 lbs.

There are all sorts of ways to go, you can't go wrong if you spend some time finding what is comfortable for you - for me, at $95 I think the Kelty MAP has been a good choice.