I've been looking at these things online, and some of them talk about being rain/wind/snow proof, and others say that they're suppose to be used with a tarp, tent, etc.

I already have a eureka zeus solo tent, and it was the lightest freestanding tent I could find for the money, which is why I bought it. I've done a fair amount of bicycle touring, and some overnight hikes, but the weight of some of the bivies I've seen is 1/2 to 1/3 the size and weight of my tent. I plan on hitch hiking to alaska this year (at least attempting to), and I'd like to save as much weight as possible obviously.

I won't be doing any week long wilderness hikes or anything, but I will be camping pretty much every night to save money. So are the cheaper bivies (under $150) durable enough and weather resistant enough that they'd be worth buying and taking over the tent I already have? Some of the descriptions I've read on a few of the bivies give me the impression that they have no form of netting to keep out bugs and whatnot too. I'm not real big on having insects crawling on my face all night, so this is another concern, and if bivies are only meant to be emergency shelters or short term minimalist shelters for people who are into roughing it more than I am, then that probably won't work for me either.

Also, if a bivy is capable of what I want to do, can someone reccomend a good cheap one? Thanks.