Make your decision with a few factors in mind.
Do you consider yourself to be a mobile angler, prepared to move with a change in wind direction or on seeing fish activity, or are you the bait and wait type? If you are the former a lightweight bivvy, or even a good brolly system, is likely to be desireable. If you are the latter, weight might not be a particularly significant factor.
Are you mad enough to be out in sub-zero conditions? Something with a twin skin might be preferable if it's a regular thing for you.
Are you likely to take it abroad, especially in summer? Something with vents may be a good idea.
Are you one of the unfortunates that gets bitten to death by insects in the height of summer? Look to something with mossie net doors rather than open fronts.
Do you need masses of room? Have a huge bedchair or masses of gear? Might be worth looking at the bigger end of the market.
Many of the features that modern bivvys have come down to personal preference. Some folks simply can't do without a peak of some kind, others don't care. Some want a bivvy to be built to withstand a hurricane, to many that doesn't really matter.
I have two different setups to cover as many angles as possible. A big, heavy, borderline indestructable, twin skin bivvy, for when I expect dodgy weather, and a brolly system for better weather and more mobility.
It's a little hard to give specifics, not many folks get to know the ins and outs of all the multitude of options, and I'm no exception. However, on a budget, for an all singing twin skin option with stacks of options and decent space, I've been impressed with my mates Chub Tri-Brid. It can be stripped right back to a simple pram hood open shelter if you want. He's taken to using it in preference to a much more expensive Fox bivvy that he owns. Even with the wrap, it can be had for less than your budget.
For a lighter, quicker option, I can recommend the Fox Supa Brolly. I tend to use mine (just the brolly, not the full blown system) in spring and summer. However, as I suffer with bitey things, I had to buy the optional mossie net front. Even with the additional mossie net front, it'll still come in just under budget. I happily used it this weekend knowing that despite the overnight frost, there wasn't going to be any extreme weather.