If you're buying online you need to find a reputable site which might be difficult, because jade bangles don't appeal as much to the Western market, so sources are probably limited. You're also likely to pay over the odds for a bangle, due to the increased costs of exporting and adding another middleman. Only thing I can recommend is to look at
a lot of bangles to get an idea of what they normally go for, and haggle when you see something you like.
There's a Chinese proverb which says "you can put a price on gold & silver, but jade is priceless" and it's true - it's very difficult to put a price on jade. Therefore as long as you find something you really love within your budget, it's worth it.
A haggling tip I picked up

Even if you're in love with a bangle, you absolutely must pick out its flaws e.g. it's a little too big/small, too green/not green enough, you can see cracks or lines or other imperfections inside of the bangle, etc. And rather than coming up with a random price, ask them what is the lowest price they're willing to sell at, then knock it down from there. It really helps if you know what you're talking about, but if you don't know, act like it