Generally, I'd suggest a down bag as you get the most warmth for the weight. One of the absolutely lightest bags around is the Western Mountaineering Megalite ($280) which weighs a mere one pound, nine ounces, yet is rated to 30 degrees Fahrenheit. That company's Highlite ($220) is even lighter - one pound even. It's rated to 40 degrees, but you'd probably be fine down to freezing provided you layer up just a little with clothes I expect you'll have on hand anyway. And that would be my advice - think of the bag as an adjunct to other stuff you'll have, not as the sole sleeping insulation. Other good, light bags include the Marmot Hydrogen (one pounds, seven ounces; 30 degrees; $249) and the Feathered Friends' Vireo (one pound, 45 degrees; $220 with water-repellent Epic shell).
The waterproofing bit is hard. Because of their seams, nearly all bags leak regardless of the shell. Best, I think, to pack a sheet of thin polyethylene sheeting (buy it at a hardware store) and wrap yourself in that if need be. Otherwise, by the time you start fussing with commercial sleeping bag covers and that sort of thing, you've added one or two pounds of weight, and $150 or more in cost. The cost may not matter, as you indicate, but I'm sure the weight does.
Hope that helps. Good luck!
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