Need more information. Where are you heading to? Without knowing the temperatures you’ll be enduring I’m hesitant to recommend any kit less it be wrong & put you in unnecessary discomfort, not that Im normally that caring.
I’ve got a Rab Survival Zone that has been used on and off for the past 14 years but if I was buying a cheap one now it’d be the Alpkit one, spending more then I’d want one with a sleave for a thermarest.
You’ll be looking at something like the Rab Explorer series, possibly a 500 stretching to a 700 if there is a likelihood of below freezing temps.
He’s asking about bivvy bags not sleeping bags.
Just buy an AlpKit Hunka XL, think of it as a sleeping bag cover and you’ll be fine. If you want something you can fully shut yourself up in (try not to if possible) then a RAB Storm would be about your cheapest option.