A few Christmas's ago my wife and I put together a BOB for her sister as a gift. I sourced most of the stuff from CampingSurvival - here's the list (a friend of my sister-in-law asked for it so I still have a copy of it):
AMK Optimist Kit Adventure Medical Kits 16.01
Kaito Small Pocket Portable AM/FM Radio 6.49
5-Piece Mess Kit 4.99
Potable Aqua Water Purification Tablets 5.94
Coghlans Emergency, Survival & Camp Stove 4.18
Coghlans Fuel Tablets 1.91
Coghlan's 5 in 1 Survival Aid. Includes Match Box, Compass, Signal Mirror, Whistle, Fire Starter 1.82
Coghlan's Braided Nylon Cord 2.26
Coghlans Multi-function Campers Tool 5.49
10' X 12' Poly Tarp 7.70
Equinox Traveler Fleece Blanket 16.00
Tube Tent 5.99
MAINSTAY 3600 Calorie Emergency Food Rations 6.14
Black One-Hole Face Mask 2.69
Coghlan's LIGHTWEIGHT PONCHO 3.22
Cow Hide Working Gloves With Header 1.95
U.S. Navy Wool Ski Socks 5.43
Emergency Solar Survival Sleeping Bag 3.49
Nalgene 32 oz. Wide Mouth Lexan Bottle Gray / Blue 15.08
Coghlan's LED Cliplight - Head Lamp 13.98
Coghlan's Sewing Kit 1.47
Subtotal for Campingsurvival.com: 132.23
The list isn't complete - some other items I included of course were:
1) backpack (which you have)
2) sheath knife
Besides everything above there's a few more items/suggestions we added to her BOB:
Dust mask
Baby-wipes in a Ziploc bag
Big freezer-size Ziploc bags for waterproof storage of all the gear
Ten AAA batteries
Tinfoil
Scissors
Tealight candles (at least five) (you might consider this:
http://www.campingsurvival.com/ucomicala.html)
Strike-anywhere matches (to be put in your 5-in-1 Survival Aid)
At least two Bic lighters
Lots of Tea - Green, Black, Herbal, whatever you prefer (or coffee!)
A few multi-vitamins
Umbrella
Good walking shoes
Hat (baseball cap)
Bandanna
Phone numbers for Friends and Family
Notepad and Sharpie
Sunscreen
Bug spray
Toiletries (toilet paper, toothbrush, toothpaste, soap, etc.) And don't forget Dental Floss!!
Duct Tape
Safety Pins
Trash Bags - at least two large ones
Deck of playing cards and/or lightweight book to read
And of course water for the two Nalgene bottles (you should carry at least 2 liters of water, preferably 4)
Map
And of course a wad of money - preferably paper bills of low denomination, ones, five, tens maybe some twenties.
In general, any clothes that you pack should not be cotton - preferably a synthetic like polypropylene, nylon, or polyester. Cotton which becomes wet from rain or perspiration does not dry readily and can promote hypothermia. Wool is ok. Obviously this is not much of a consideration during the summer months.
You should also print out the following resource and follow its recommendations:
http://www.ncua.gov/Publications/brochures/EmergencyFinancialFirstAidKit.pdf
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Just the above will run a bit over $160. Many ready-made go-bags on the internet will cost well under $100 - but they are woefully inadequate (though better than nothing). Everything above I believe is designed to be cheap but the minimum requirement to be adequately prepared. But if you do wish to spend more money the items to first considering upgrading are (in order):
1)Backpack - you very well might have to haul all this stuff around and it's a lot easier with a quality backpack.
Hydration packs are great because your water carrier is integrated.
(if you're going to get a sleeping bag and tent you'll need a bigger, full-size backpack)
2)Gore-tex Jacket - a cheap vinyl poncho is ok - a breathable, waterproof jacket is much, much better. Exposure to the elements will kill you faster than almost anything else.
It doesn't have to be the Gore-Tex brand - the patent has run out so any jacket with a Breathable, Waterproof Laminate is essentially Gore-Tex technology.
There's lots of options out there - check out Sierra Trading Post.
3)Tent - a $6 tube tent will help ward off hypothermia but it doesn't offer the same comfort as an actual tent.
Campingsurvival.com has cheap options:
http://www.campingsurvival.com/cam2mantrail.html
Getting a quality tent is a good idea. It's your home-away-from-home and you won't have to take out a mortgage to afford it! Some discounted options are available at SierraTradingPost (a great place to shop for camping gear). Most tents at SierraTrading do not come with footprints but you can cut one out of a blue poly tarp. Consider weight when buying a tent - a 3 pound tent instead of a 6 pound tent means 3 more pounds of food and water that you can carry.
4)Water Filter - iodine tablets work, but it does leave the water tasting a little funny
Two options here:
a cheap straw:
http://www.scoutgear.com/as713215.html
or an expensive MSR filter (which integrates with the Nalgene bottles):
http://www.theoutdoorworld.com/products2.cfm?id=35151
5)More food - 3600 calorie supply is going to leave you pretty hungry at the end of 3 days
You could just order two of the Mainstay emergency food rations.
MREs are heavy and bulky but if you have room in your apartement it's nice to have a box around:
http://www.campingsurvival.com/fulmremilrea.html
Another option is freeze-dried camping foods:
http://www.campingsurvival.com/moho1brbafo.html
Or just a plastic jar of Peanut Butter works great! Hard candy is nice to have. Food supplies don't have to be fancy - a ziploc bag full
of oatmeal will carry you a long way.
6)Upgraded medical kit - Adventure Medical is a great brand, shop around on Froogle.com for prices and options
http://froogle.google.com/froogle?q=adventure+medical&btnG=Search+Froogle
7)Sleeping Bag/Sleeping Pad - a tent will keep hypothermia away but shivering on the cold hard ground is still unpleasant
A sleeping pad is at least as, if not more, important than a sleeping bag. The ground is a huge heat sink that will draw off your body's heat as you lie on it. Sleeping pads don't need to be fancy - a yoga mat works fine. If you want a nice sleeping pad and/or sleeping bag
http://www.sierratradingpost.com/ probably has some good deals.
8)A hatchet or saw to collect firewood
I'm the same way about judging size...I thought this thing was ~daypack size when I bought it. It's got a ton of bells and whistles that aren't mentioned anywhere that I read.